We believe that Mary went to the hallway. It is probably not the case that Brian is a rhino. It is certain that John put down the apple. There is little chance that if 'John put down the apple and Mary went to the hallway' then Bill got the milk there. It is probably not the case that if either 'John put down the apple' or 'Brian is a rhino' but not both then Fred left the football. It is probably not the case that if 'Brian is a rhino' or 'Mary went to the hallway' or both then Lily is a frog.
There is almost no chance that either 'Fred left the football' or 'Brian is a rhino' but not both.
Chances are about even that either 'Fred left the football' or 'Brian is a rhino' but not both.
There is almost no chance that either 'Fred left the football' or 'Brian is a rhino' 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
  0.75::factA.
  0.25::factB.
  1.0::factC.
  %hop1
  0.1::factX:-and(factC,factA).
  0.25::factY:-xor(factC,factB).
  0.25::factZ:-or(factB,factA).

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

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