It is certain that Fred left the football. It is probably the case that Greg is a frog. It is probably the case that Emily is a cat. It is improbable that if 'Greg is a frog' or 'Fred left the football' or both then Sandra grabbed the apple. There is little chance that if 'Greg is a frog and Emily is a cat' then Lily is a swan. It is probable that if 'Emily is a cat' or 'Greg is a frog' or both then Brian is green.
It is probably not the case that either 'Greg is a frog' or 'Fred left the football' but not both.
It is probably not the case that either 'Greg is a frog' or 'Fred left the football' but not both.
It is probable that either 'Greg is a frog' or 'Fred left the football' 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
  1.0::factA.
  0.7::factB.
  0.7::factC.
  %hop1
  0.1::factX:-or(factB,factA).
  0.1::factY:-and(factB,factC).
  0.7::factZ:-or(factC,factB).

  %hop2
  conclusion:-xor(factB,factA).

  query(conclusion).
  
Either 'Greg is a frog' or 'Fred left the football' but not both.