Chances are about even that John dropped the milk. Chances are slight that Brian is a swan. Chances are slight that Mary went to the office. It is impossible that if 'John dropped the milk' or 'Brian is a swan' or both then Greg is a rhino. There is almost no chance that if 'Mary went to the office and Brian is a swan' then Fred discarded the apple. We doubt that if 'John dropped the milk' or 'Brian is a swan' or both then Lily is a lion.
We believe that 'Greg is a rhino' or 'Mary went to the office' or both.
There is little chance that 'Greg is a rhino' or 'Mary went to the office' or both.
We believe that 'Greg is a rhino' or 'Mary went to the office' 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
  0.5::factA.
  0.1::factB.
  0.1::factC.
  %hop1
  0.0::factX:-or(factA,factB).
  0.02::factY:-and(factC,factB).
  0.2::factZ:-or(factA,factB).

  %hop2
  conclusion:-or(factX,factC).

  query(conclusion).
  
'Greg is a rhino' or 'Mary went to the office' or both.