It is highly likely that Greg is a lion. It is impossible that Lily is a lion. There is little chance that Mary went to the office. Chances are about even that if 'Mary went to the office' or 'Lily is a lion' or both then Emily is a cat. It is unlikely that if 'Greg is a lion and Lily is a lion' then Fred dropped the apple. It is unlikely that if 'Greg is a lion and Mary went to the office' then Brian is gray.
It is probably not the case that either 'Greg is a lion' or 'Brian is gray' but not both.
It is highly likely that either 'Greg is a lion' or 'Brian is gray' but not both.
It is probably not the case that either 'Greg is a lion' or 'Brian is gray' 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
  0.9::factA.
  0.0::factB.
  0.1::factC.
  %hop1
  0.5::factX:-or(factC,factB).
  0.2::factY:-and(factA,factB).
  0.2::factZ:-and(factA,factC).

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

  query(conclusion).
  
Either 'Greg is a lion' or 'Brian is gray' but not both.