in furtherance of disclosing important features of the present invention and distinguishing it from the references of prior art , a preliminary overview of related concepts and prior art work is in order . in the realm of pure mathematics , operations on real numbers are functions that have a natural domain . for example , the square root of a negative real number is not defined , so the natural domain of the square root operation is the set of all non - negative real numbers . division by zero is also not defined , so the natural domain of the reciprocal operation is the set of all non - zero real numbers . and so on for each contemplated operation . in other words , whenever the input to an operation is not an element of the natural domain of the operation , the operation is not defined . continuity is another important property of functions of real numbers . formal definitions of a continuous function are well - known in the prior art and can be given in terms of sequences or limits of the function . informally , if a function is continuous then any time a sequence converges in the domain , the image of the sequence in the range also converges . in other words , one could either take the limit first , and then apply the function , or apply the function first , and then take the limits . even more particularly , if ƒ : r n → r is a function that maps n - dimensional real vectors r n to real numbers r and d ƒ ⊂ r n is the natural domain of ƒ , then for any x ⊂ d ƒ , the property of continuity may be further defined in terms of the restriction of ƒ on x . in this case , the only relevant aspect of the continuity property is with respect to the portions of the function ƒ that are restricted to the domain x , wherein x is a subset of the natural domain d ƒ of the function . for example , consider the function floor ( x ): r → r depicted in fig1 which rounds the real number x to the closest integer n such that n ≦ x . the floor function is defined for any real number x . so the natural domain of the function is the entire real number line . however , the function is not continuous on its natural domain . this is because , for example , if x = 1 there is no sequence or limit from the left of x = 1 that converges to ƒ ( x )= 1 . however , if the restriction of the floor function to the interval x =[ 1 , 3 / 2 ] is instead considered , then the floor function is continuous on x because there is no xεx such that x & lt ; 1 and this means there can be no sequence or limit from the left of x = 1 . the restriction of a function to x can be continuous if and only if x is also a subset of the natural domain of the function . for example , the natural domain of the square root operation is the set of all non - negative real numbers . so the restriction of the square root operation to x =[− 16 , 4 ] cannot be continuous because there are elements of x =[− 16 , 4 ] which do not belong to the natural domain of the operation and for which the operation is not defined . in other words , x in this case is not a subset of the natural domain of the operation . because the empty set ( ø ) is a subset of every set , it is also a subset of all natural domains of all functions . the restriction of a function to the empty set must therefore be contemplated . as it turns out , the restriction of any function to the empty set x = ø is always defined and continuous . this is perhaps a little counterintuitive , but nonetheless mathematically correct . formal mathematical reasoning to prove this is well known in the prior art . the present invention is concerned with modal interval arithmetic and modal interval analysis . in particular , the present invention is concerned with an improved system and method of detecting exceptional conditions in modal interval operations . the prior notions of a real function , the natural domain of a real function , and the restriction of a real function to a subset of the natural domain of the function are therefore lifted into the topic of modal intervals . for the purposes of the present invention , a modal interval is defined as an ordered pair [ a , b ] such that a and b are real numbers or signed infinities . however , the two pairs [−∞, −∞] and [+∞, +∞] are excluded . note that no restriction a ≦ b is required . and the notation xε [ a , b ] may be used as an abbreviation for xεset ([ a , b ]). the empty set ( ø ) is not a modal interval , however set ( 0 )= 0 . a closed interval includes all of its limit points . every modal interval [ a , b ] is a closed interval . if both a and b are real numbers , the modal interval is bounded . if a modal interval is not bounded , then at least one of the endpoints is −∞ or +∞ and the modal interval still contains all of its limit points but not all of its endpoints . modal intervals of the form are therefore understood to be unbounded . despite the use of square brackets , infinity is never an element of any modal interval , and a modal interval [ a , b ] is called proper if a ≦ b . the modal interval is called improper if a ≧ b . it is called a point or a point - wise modal interval if a = b . note that a point - wise modal interval is also a proper and an improper modal interval at the same time . the set of all bounded modal intervals can be visualized as points in the r 2 plane , where canonical abscissa and ordinate are defined respectively as the left and right bound of a modal interval [ a , b ], i . e ., fig2 is such a visualization . points above the 2 = p line are proper modal intervals and points below this line are improper modal intervals . points on the line are point - wise modal intervals . it should be noted the subset of bounded modal intervals visualized in the figure is the famous set known in the prior art as the kaucher intervals . also , the subset of the kaucher intervals on or above the λ = ρ line is the famous set of “ classic ” intervals popularized in the late 1960 &# 39 ; s by ramon e . moore . the set of classic intervals does not include any improper intervals . as of this date , the subset of bounded and unbounded proper modal intervals is the set of intervals supported in the working draft of the ieee standard for interval arithmetic . that standard is currently under development by the p1788 working group committee at the time of this writing and does not provide any provision for bounded or unbounded improper intervals . the present invention therefore contemplates the broadest set of “ intervals ” as it is the only one that consists of a set wherein each element of the set may be bounded or unbounded , proper or improper . a predicate is a boolean function , and a proposition is a predicate wherein each variable is universally (∀) or existentially (∃) quantified . the modal quantifier q of a modal interval [ a , b ] quantifies a real variable x by the definition q ( x ,[ a , b ])= if a ≦ b then ∀ x εset ([ a , b ]) else ∃ x εset ([ a , b ]). and the corresponding modal quantifiers d , e and u are defined as with the modal quantifiers q , d , e and u it is possible to form propositions with modal intervals . for example , if a and b are modal intervals then the inclusion ( c ) relation is a ⊂ b d ( a , a ) q ( b , b ): a = b . a ≦ b u ( a , a ) e ( b , b ): a ≦ b and u ( b , b ) e ( a , a ): a ≦ b . fig3 is a geometric visualization of these modal interval relations in the r 2 plane . the inclusion and less - or - equal relations are shown for modal intervals b , c , d and e relative to the modal interval a . if x is a real number , the rounding operators ∇( x ) and δ ( x ) are digital approximations of x such that the relations inn ([ a , b ])=[ δ ( a ),∇( b )] and out ([ a , b ])=[∇( a ), δ ( b )] are the “ inner ” and “ outer ” digital roundings , respectively of [ a , b ]. the inner and outer digital roundings are universally possible for any digital scale and satisfy the property a decoration is a mathematical property of a real function restricted to the domain of its modal interval inputs . decorations provide a framework for detecting exceptional conditions such as out - of - domain arguments or non - continuous functions . looking at the syntactic tree for a real function , where the nodes are operators , the leaves are variables , and branches define the domain of each operator , the real function can be operationally extended to a modal interval expression by using the computational program implicitly defined by the syntactic tree of the real function . this is accomplished by transforming all of the real operators into their modal interval extension and all of the real variables into modal interval variables . fig4 is an example that shows how the syntax tree of the real function implicitly defines a modal interval expression . real operators and variables 12 are respectively transformed into their modal interval counterparts 15 . evaluation of the expression begins at the leafs of the tree , where variables are propagated up the branches to the operations . the operations accept the variables as operands , perform an operation and generate a result . the result of each operation is then propagated up the branches into other operations until all nodes in the tree have been evaluated and a final result is propagated to the root of the tree . the present invention makes a distinction between two types of decorations . a static decoration is the absolute mathematical truth about the restriction of an individual operation to the domain of its modal interval inputs . a tracking decoration , on the other hand , is a mathematical implication that is obtained for a modal interval expression by propagating static decorations in the expression tree up the branches and to the root . the method used to propagate static decorations through an expression tree in order to obtain a tracking decoration is called property tracking . fig6 is a venn diagram showing the logical relations between the five sets of tracking decorations . the five tracking decorations form the inclusion relations note that ein is a subset of all tracking decorations and gap is a superset of all tracking decorations . for example , if ƒ is the square root operator and x =[ 1 , 4 ] then for this particular ( ƒ , x ) pair the restriction of ƒ on x is defined and continuous . since x is not empty , the ( ƒ ; x ) pair cannot be an element of ein . additionally , the ( ƒ , x ) pair cannot be an element of ndf because in this case ƒ ( x ) is not empty , either . however , if x is the empty set then the ( ƒ , x ) pair is an element of ein , and since ein is a subset of all tracking decorations the ( ƒ , x ) pair is also an element of dac , def , gap and ndf . an important note should be made regarding the prior art as it pertains to decorations . a draft of the ieee standard for interval arithmetic , which is under development at the time of this writing , contains a decoration system that was at least partially conceived by the applicant of the present invention . that decoration system has some common characteristics to the decoration system of the present invention , such as def and dac decorations . however , there are some important differences , namely the absence of an ein decoration as well as the inclusion of several other decorations not defined in the present invention such as ill ( ill - formed ), com ( a common interval ) and bnd ( a bounded interval ). applicant &# 39 ; s white paper entitled “ decorations as state machine ” provides rationale why these competing decoration systems are unnecessary or insufficient for reliable interval computations and how the unique decoration system of the present invention overcomes those issues . for any ( ƒ , x ) pair , the notation s ( ƒ , x ) indicates which static decoration the ( ƒ , x ) pair is an element of as a function of table 2 . if ƒ has n operands , the notation s ( ƒ , x ) is shorthand for s ( ƒ , x 1 , x 2 , . . . , x n ). if ƒ : r n → r is a real function and d ƒ ⊂ r n is the natural domain of ƒ , and if x is an n - dimensional modal interval box , then table 3 in fig5 is an alternate definition of s ( ƒ , x ) that defines the same sets as table 2 in fig5 . fig7 is a venn diagram showing the logical relations between the five sets of static decorations . while the tracking decorations in table 1 of fig5 partition the universe of all ( ƒ , x ) pairs into a hierarchical arrangement of nested sets ( c . f . fig6 ), the static decorations in tables 2 - 3 of fig5 partition the universe of all ( ƒ , x ) pairs into five disjoint sets ( c . f . fig7 ). for example , if ƒ is the square root operation and x 1 =[ 2 , 4 ], x 2 =[− 1 , 1 ] and x 3 =[− 4 , − 2 ] then s ( ƒ , x 1 )= dac because set ( x 1 ) is a nonempty subset of d ƒ and the restriction of ƒ on x 1 is continuous ; s ( ƒ , x 2 )= gap because set ( x 2 ) is not a subset of d ƒ but the intersection of set ( x 2 ) and d ƒ is nonempty ; and s ( ƒ , x 3 )= ndf because x 3 is not empty but the intersection of set ( x 3 ) and d ƒ is empty . property tracking is the method used to propagate static decorations through an expression tree in order to obtain a tracking decoration . to facilitate this method , the notion of a decorated interval is contemplated . a decorated interval is a pair ( x , d ) that consists of a modal interval x and a tracking decoration d . the empty set is not an interval , however the notion of a decorated empty set ( ø , d ) is also contemplated . the method of property tracking begins with initialization . each modal interval variable x 1 , x 2 , . . . x n in the leafs of the expression tree is promoted to a decorated interval ( x 1 , dac ), ( x 2 , dac ), . . . , ( x n , dac ). if any variable in a leaf of the expression tree is an empty set , then the empty variable is promoted to a decorated empty set ( ø , ein ). similarly , if any variable in a leaf of the expression tree is [+∞, +∞] or [−∞, −∞], then the variable is promoted , respectively , to ([+∞, +∞], gap ) or ([−∞, −∞], gap ). the decorated variables are then propagated up the branches of the expression tree to the operations . the operations accept the decorated variables as operands , perform an operation and generate a decorated result . for each operation , if ƒ : r n → r is the real function and ( x i , d i )=(( x 1 , d 1 ),( x 2 , d 2 ), . . . , ( x n , d n ) are the decorated interval operands of ƒ , then the decorated result of the operation has the tracking decoration t ( ƒ ,( x i , d i ))= min { s ( ƒ , x 1 , x 2 , . . . , x n ), d 1 , d 2 , . . . , d n }. in other words , the decorated result of the operation has a tracking decoration which is the minimum element of a set formed by the union of the static decoration s ( ƒ , x 1 , x 2 , . . . , x n ) of the operation and the decorations d 1 , d 2 , . . . , d n of the operands . for the sake of determining the minimum element of a set of decorations , the decorations are linearly ordered the decorated result of each operation is then propagated up the branches into other operations . the procedure is repeated until all nodes in the tree have been evaluated and a final decorated result is propagated to the root of the tree . inside a computer , the endpoints of a modal interval may be represented by elements of a digital scale . in a preferred embodiment of the present invention , the digital scale conforms to the binary64 interchange format encoding of ieee 754 - 2008 . for the sake of discourse , only the binary64 encoding will be explained or considered in the rest of this document . however , the use of other digital scales or interchange format encodings as it pertains to the present invention should be obvious . a binary64 floating - point datum is 64 bits of information partitioned into sign , exponent and fraction fields as shown in register 20 of fig8 . the fraction field is in bits 0 to 51 ; the exponent field is in bits 52 to 62 ; and the sign field is the most significant bit 63 . the floating - point data represented by this encoding is : zero . if the exponent and fraction are zero , then the floating - point data is a positive or negative zero + 0 or − 0 when the sign bit is 0 or 1 , respectively . non - zero finite number . if the exponent and fraction are not zero and the exponent bits are not all 1 , then by definition the floating - point data is a positive or negative non - zero finite number when the sign bit is 0 or 1 , respectively . infinity . if the exponent bits are all set to 1 and the fraction is zero , then the floating - point data is +∞ or −∞ when the sign bit is 0 or 1 , respectively . the infinities are the maximum numbers that can be represented in floating - point format . negative infinity is less than any finite number and positive infinity is greater than any finite number . nan ( not - a - number ). if the exponent bits are all set to 1 and the fraction is not zero , then the floating - point data is a non - number that lies outside the range of representable floating - point numbers , regardless of the sign bit . if bit 51 is set to 1 , then the nan is quiet ( qnan ); otherwise the nan is signaling ( snan ). for the purposes of the present invention , bits 49 to 50 are the nan decoration field . the nan decoration field is not part of the ieee 754 - 2008 standard and is defined solely for the sake of the present invention . a binary64 datum may be associated with a truth table 23 consisting of a set of classification bits as depicted in fig9 . the classification bits consist of a sign ( s ), nan ( n ), infinity ( i ) and zero ( z ) bit . bit 63 of the binary64 datum is copied to the sign bit . the nan , infinity and zero bits are set to 0 or 1 depending on the combined value of the exponent and fraction of the binary 64 datum . if the combined value is greater than 0x7ff0000000000000 , then the nan bit is set to 1 ; if the combined value is equal to 0x7ff0000000000000 , then the infinity bit is set to 1 ; if the combined value is equal to zero , then the zero bit is set to 1 ; otherwise the respective nan , infinity and zero bits are set to 0 and the binary64 datum is a positive or negative non - zero finite number . if x is a floating - point datum , then s ( x ), n ( x ), i ( x ) and z ( x ) are notations used in this document to represent the respective values of the sign , nan , infinity and zero classification bits of truth table 23 . if bit 51 of a nan is set to 1 , then the nan is quiet ( qnan ); otherwise the nan is signaling ( snan ). if the nan is signaling , at least one other fraction bit of the nan must be set to 1 to distinguish the nan from an infinity . the difference between a quiet and signaling nan is for the sake of compatibility with ieee 754 - 2008 standard interchange format encoding . however , the present invention does not require signaling nan operations . bits 0 to 50 are the nan “ payload .” all bits of the payload may be set to any value so long as the entire fraction field of a nan does not become zero . the preferred embodiment of the present invention may use bits 49 to 50 of a nan payload as a nan decoration field to encode a representation of a tracking decoration . if x is a nan , then t ( x ) is the representation of a tracking decoration encoded within the nan decoration field of x as depicted in table 4 of fig5 . the tracking decoration ein has no designated encoding within the nan decoration field . the reason for this will be shown in the following parts of the document . if n is the representation of a tracking decoration , then nan ( n ), qnan ( n ) and snan ( n ) are notations used in this document to represent the respective encodings of a nan , quiet nan or signaling nan when the bits of the decoration field are set to the corresponding value of n as depicted in table 4 of fig5 . a modal interval datum [ a 1 , a 2 ] may be encoded into 128 bits as two binary64 datums a 1 and a 2 as illustrated in register 27 of fig1 . if a 1 or a 2 is a nan , or if a 1 and a 2 are infinities with the same sign , then the 128 - bit encoding represents non - interval data . all non - interval data is an encoding of a decorated empty set . if a 1 and a 2 are binary64 datums that do not represent a nan , and if n is a representation of one of the tracking decorations dac , def , gap or ndf ( see table 1 in fig5 ) encoded in a nan decoration field as depicted in table 4 of fig5 , then a decorated empty set ( ø , n ) is encoded by any non - interval data of the form [ a 1 , nan ( n )] or [ nan ( n ), a 2 ]. any such encoding of a decorated empty set ( ø , n ) may be provided as input to an operation of the present invention , and a canonical encoding may be provided as a result of an operation which produces a decorated empty set as output . non - interval data of the form is an encoding of the decorated empty set ( ø , ein ), and non - interval data of the form fig1 shows how a modal interval datum [ a 1 , a 2 ] is classified by an empty ( e ) bit and a representation of a tracking decoration ( t ). the classification 33 is a function of truth tables each consisting of the sign ( s ), nan ( n ) and infinity ( i ) bits , respectively , of the binary64 datums a 1 and a 2 , as well as the respective nan decoration field of a 1 or a 2 , if either a 1 or a 2 is a nan . the logic diagram 30 computes the classification 33 according to the following specifications . the empty bit in fig1 is set to 1 if a 1 and a 2 are both infinities with the same sign or if a 1 or a 2 is a nan ; otherwise the empty bit is set to 0 . if a 1 and a 2 are infinities with the same sign , the tracking decoration is gap ; if a 1 is a nan and a 2 is not , the tracking decoration is a representation of the nan decoration field from a 1 ; if a 2 is a nan and a 1 is not , the tracking decoration is a representation of the nan decoration field from a 2 ; otherwise the tracking decoration is ein . strictly speaking , the tracking decoration has no meaning unless the empty bit is set to 1 . if the empty bit is 0 , the modal interval datum is a bounded or unbounded modal interval and the tracking decoration is not used . if [ a 1 , a 2 ] is a modal interval datum , then e ( a 1 , a 2 ) and t ( a 1 , a 2 ) are notations used in this document to represent the respective values of the empty bit and the tracking decoration . the present invention provides an improved system and method for reliable and efficient modal interval operations using decorations . the preferred embodiment of the present invention is an arithmetic functional unit ( afu ) as depicted in applicant &# 39 ; s u . s . pat . no . 7 , 949 , 700 entitled “ modal interval processor .” modal interval operand and result signals for the afu are digitally encoded using the methods described in the previous section of this document entitled “ digital encodings of modal intervals and decorations .” as will be shown subsequently for select modal interval operations , the result of a modal interval operation is typically obtained by performing a floating - point calculation on select endpoints of the modal interval operands . because floating - point calculations are often inexact , the present invention requires the rounding operators ∇( x ) and δ ( x ) to ensure modal interval results obey the “ outer ” digital rounding of modal intervals . fig1 shows a unary modal interval operation for example computes in a functional element 36 . element 36 may comprise one or more sub - elements and may be implemented as hardware , firmware or software . element 36 takes an operand [ a 1 , a 2 ] held in register 27 as input and produces a result [ b 1 , b 2 ] held in a result register 39 . fig1 is a more detailed logic diagram of fig1 . a truth table 48 specifies the value in result register 39 . the truth table is a function of the empty ( e ) bit 42 associated with operand 27 . a truth table value of 0 selects the computation function associated with the particular operation . the other truth table value specifies the result 39 is a copy of operand 27 . therefore , if [ a 1 , a 2 ] is an encoding of non - interval data , i . e ., if [ a 1 , a 2 ] is an encoding of a decorated empty set , then the operand [ a 1 , a 2 ] is the result of the operation . otherwise [ a 1 , a 2 ] is an encoding of a bounded or unbounded modal interval and the result is defined separately for each operation . fig1 - 24 depict the result of several unary modal interval operations after a determination has been made that the operand [ a 1 , a 2 ] in fig1 is an encoding of a bounded or unbounded modal interval . fig1 is a logic diagram for a modal interval negation operation . the operation effectively multiplies the modal interval by − 1 . since negation of floating - point numbers is exact , the modal interval negation operation is exact and requires no rounding operators . fig1 is a logic diagram for a modal interval reciprocal operation . the operation is defined if and only if a 1 and a 2 are both non - zero numbers with the same sign , otherwise the result of the operation is a decorated empty set . if a 1 and a 2 are both zero , the empty set is decorated with ndf ; otherwise the empty set is decorated with gap . fig1 is a logic diagram for a restricted modal interval reciprocal operation . the operation silently removes zero from the input of the operation . unlike the modal interval reciprocal operation in fig1 , the operand [ a 1 , a 2 ] may have one endpoint that is zero . fig1 is a logic diagram for a modal interval sign operation . the operation is defined for the entire real number line . however , the operation is continuous if and only if a 1 and a 2 are both strictly positive , both strictly negative or both zero . if the operation is not continuous , the result is an encoding of the decorated empty set ( ø , def ). fig1 is a logic diagram for a modal interval absolute value operation . since absolute value of a floating - point number a 1 or a 2 is exact , the modal interval absolute value operation is exact and requires no rounding operators . fig1 is a logic diagram for a modal interval square operation . fig2 is a logic diagram for a modal interval square root operation . the operation is not defined if a 1 or a 2 is a negative non - zero number . in conformance to the ieee 754 - 2008 standard , this allows − 0 to be an element of the natural domain of the operation . if a 1 or a 2 is a negative non - zero number , the result of the operation is a decorated empty set . if a 1 and a 2 are both negative non - zero numbers , the empty set is decorated with ndf ; otherwise the empty set is decorated with gap . fig2 is a logic diagram for a restricted modal interval square root operation . the operation silently removes negative non - zero numbers from the input of the operation . unlike the modal interval square root operation in fig2 , the operand [ a 1 , a 2 ] may have one negative non - zero number as an endpoint , so long as a 1 and a 2 are not both negative non - zero numbers . fig2 is a logic diagram for a modal interval exponential operation . fig2 is a logic diagram for a modal interval logarithm operation . the operation is defined if and only if a 1 and a 2 are both non - zero positive numbers . if a 1 or a 2 is less - or - equal to zero , the result of the operation is a decorated empty set . if a 1 and a 2 are both less - or - equal to zero , the empty set is decorated with ndf ; otherwise the empty set is decorated with gap . fig2 is a logic diagram for a restricted modal interval logarithm operation . the operation silently removes negative numbers and zero from the input of the operation . unlike the modal interval logarithm operation in fig2 , the operand [ a 1 , a 2 ] may have one endpoint less - or - equal to zero , as long as a 1 and a 2 are not both less - or - equal to zero . fig2 shows a binary modal interval operation for example computes in a functional element 37 . element 37 may comprise one or more sub - elements and may be implemented as hardware , firmware or software . element 37 takes operands [ a 1 , a 2 ] and [ b 1 , b 2 ] held respectively in registers 27 and 28 as input and produces a result [ c 1 , c 2 ] held in a result register 29 . fig2 is a more detailed logic diagram of fig2 . a comparator 44 receives the tracking decoration bits derived from the operands 27 and 28 and then provides a 1 output when the three tracking bits t ( a 1 , a 2 ) are numerically larger than the three tracking bits t ( b 1 , b 2 ), and a 0 output otherwise . logic elements 50 produce a truth table 58 that specifies the value in result register 29 . a truth table value of 0 0 selects the computation function associated with the particular operation . a truth table value of 1 1 selects a value [ d 1 , d 2 ] produced by multiplexing operands 27 and 28 based on the output of comparator 44 . the other truth table values specify the result defined for that truth table value . the logic elements 50 are a function of the empty ( e ) bit and tracking decoration ( t ) pairs 42 and 43 associated respectively with operands 27 and 28 . if [ a 1 , a 2 ] or [ b 1 , b 2 ] is an encoding of non - interval data , i . e ., if [ a 1 , a 2 ] or [ b 1 , b 2 ] is an encoding of a decorated empty set , the result of the operation is one of the operands [ a 1 , a 2 ] or [ b 1 , b 2 ]. if both operands are decorated empty sets , then the operand with the minimum tracking decoration is the result , taking care to return operand [ a 1 , a 2 ] in the event both operands are decorated empty sets with the same tracking decoration ; if only one operand is a decorated empty set , then the operand which is the decorated empty set is the result of the operation ; otherwise [ a 1 , a 2 ] and [ b 1 , b 2 ] are both encodings of a bounded or unbounded modal interval and the result is defined differently for each operation . fig2 - 34 depict the result of several binary modal interval operations after a determination has been made that the operands [ a 1 , a 2 ] and [ b 1 , b 2 ] in fig2 are both encodings of a bounded or unbounded modal interval . fig2 is a logic diagram for a binary modal interval addition operation . the operation is not defined if a 1 + b 1 or a 2 + b 2 is a sum of infinities of opposite sign or if the sums a 1 + b 1 and a 2 + b 2 are infinities of the same sign . if the operation is not defined , the result is an encoding of the decorated empty set ( ø , gap ). fig2 is a logic diagram for a binary modal interval subtraction operation . the operation is not defined if a 1 − b 2 or a 2 − b 1 is a difference of infinities of the same sign or if the differences a 1 − b 2 and a 2 − b 1 are infinities of the same sign . if the operation is not defined , the result is an encoding of the decorated empty set ( ø , gap ). fig2 is a logic diagram for a binary modal interval multiplication operation . if a 1 , a 2 , b 1 or b 2 is an infinity or a zero , the result of the operation may require a floating - point multiplication of an infinity and a zero . according to the ieee 754 - 2008 standard , a floating - point multiplication of an infinity and a zero is an undefined operation and may return a nan . the modal interval multiplication operation deviates from this convention and instead defines the floating - point multiplication of an infinity and a zero to be according to applicant &# 39 ; s u . s . pat . no . 8 , 204 , 926 entitled “ reliable and efficient modal interval arithmetic operations .” the modal interval multiplication operation is therefore always defined so long as the operands [ a 1 , a 2 ] and [ b 1 , b 2 ] are bounded or unbounded modal intervals . fig3 is a logic diagram for a binary modal interval division operation . fig3 - 34 are logic diagrams , respectively , for the binary modal interval lattice operations minimum , maximum , meet and join . since the minimum and maximum of floating - point numbers is exact , all of the lattice operations are exact and require no rounding operators . it is possible the minimum and maximum operations may produce a result which is a representation of the decorated empty set ( ø , gap ), encoded as [−∞, −∞] for the minimum operation and [+∞, +∞] for the maximum operation .