We doubt that Brian is a rhino. It is likely that Bernhard is yellow. It is certain that Greg is white. It is unlikely that if 'Bernhard is yellow and Brian is a rhino' then Jeff moved to the garden. There is a very good chance that if 'Brian is a rhino and Bernhard is yellow' then John left the milk. There is a better than even chance that if 'Bernhard is yellow' or 'Brian is a rhino' or both then Mary went to the kitchen.
There is little chance that either 'Bernhard is yellow' or 'Brian is a rhino' but not both.
There is a better than even chance that either 'Bernhard is yellow' or 'Brian is a rhino' but not both.
There is little chance that either 'Bernhard is yellow' 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
  0.2::factA.
  0.7::factB.
  1.0::factC.
  %hop1
  0.2::factX:-and(factB,factA).
  0.8::factY:-and(factA,factB).
  0.6::factZ:-or(factB,factA).

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

  query(conclusion).
  
Either 'Bernhard is yellow' or 'Brian is a rhino' but not both.