There is little chance that Fred moved to the office. It is highly likely that Mary left the milk. Chances are slight that Brian is a frog. Chances are about even that if 'Fred moved to the office' or 'Brian is a frog' or both then Julius is a swan. We believe that if 'Mary left the milk' or 'Fred moved to the office' or both then John went to the hallway. It is probably not the case that if 'Brian is a frog and Mary left the milk' then Greg is yellow.
It is highly likely that 'Greg is yellow' or 'Mary left the milk' or both.
It is highly likely that 'Greg is yellow' or 'Mary left the milk' or both.
It is unlikely that 'Greg is yellow' or 'Mary left the milk' 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
  0.1::factA.
  0.9::factB.
  0.1::factC.
  %hop1
  0.5::factX:-or(factA,factC).
  0.75::factY:-or(factB,factA).
  0.25::factZ:-and(factC,factB).

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

  query(conclusion).
  
'Greg is yellow' or 'Mary left the milk' or both.