It is probably the case that Jeff went to the garden. It is unlikely that Bernhard is a swan. There is little chance that Mary left the apple. Chances are slight that if either 'Bernhard is a swan' or 'Mary left the apple' but not both then Julius is a lion. There is almost no chance that if either 'Mary left the apple' or 'Bernhard is a swan' but not both then Brian is green. It is unlikely that if either 'Mary left the apple' or 'Bernhard is a swan' but not both then Greg is yellow.
It is unlikely that 'Greg is yellow' or 'Bernhard is a swan' or both.
It is unlikely that 'Greg is yellow' or 'Bernhard is a swan' or both.
It is certain that 'Greg is yellow' or 'Bernhard is a swan' or 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.
  0.2::factB.
  0.1::factC.
  %hop1
  0.1::factX:-xor(factB,factC).
  0.02::factY:-xor(factC,factB).
  0.2::factZ:-xor(factC,factB).

  %hop2
  conclusion:-or(factZ,factB).

  query(conclusion).
  
'Greg is yellow' or 'Bernhard is a swan' or both.