It is probable that Emily is a mouse. There is little chance that Gertrude is a sheep. It is impossible that Greg is gray. It is almost certain that if either 'Gertrude is a sheep' or 'Emily is a mouse' but not both then John picked up the apple. It is unlikely that if 'Gertrude is a sheep' or 'Emily is a mouse' or both then Mary discarded the milk. There is a very good chance that if 'Gertrude is a sheep and Greg is gray' then Brian is white.
There is little chance that either 'Brian is white' or 'Mary discarded the milk' but not both.
There is little chance that either 'Brian is white' or 'Mary discarded the milk' but not both.
It is certain that either 'Brian is white' or 'Mary discarded the milk' but not both.

  and(A,B) :- A,B.
  or(A,B) :- A;B.
  nand(A,B) :- not(and(A,B)).
  nor(A,B) :- not(or(A,B)).
  xor(A,B) :- or(A,B), nand(A,B).

  %hop0
  0.7::factA.
  0.1::factB.
  0.0::factC.
  %hop1
  0.95::factX:-xor(factB,factA).
  0.2::factY:-or(factB,factA).
  0.8::factZ:-and(factB,factC).

  %hop2
  conclusion:-xor(factZ,factY).

  query(conclusion).
  
Either 'Brian is white' or 'Mary discarded the milk' but not both.