We doubt that Gertrude is a mouse. There is a very good chance that Greg is a swan. There is little chance that Mary discarded the milk. There is little chance that if 'Greg is a swan and Mary discarded the milk' then Bernhard is a frog. It is highly likely that if 'Mary discarded the milk and Greg is a swan' then Julius is white. It is almost certain that if 'Gertrude is a mouse' or 'Greg is a swan' or both then Brian is yellow.
It is improbable that 'Bernhard is a frog' or 'Mary discarded the milk' or both.
It is improbable that 'Bernhard is a frog' or 'Mary discarded the milk' or both.
We believe that 'Bernhard is a frog' or 'Mary discarded 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
  0.2::factA.
  0.8::factB.
  0.1::factC.
  %hop1
  0.1::factX:-and(factB,factC).
  0.9::factY:-and(factC,factB).
  0.95::factZ:-or(factA,factB).

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

  query(conclusion).
  
'Bernhard is a frog' or 'Mary discarded the milk' or both.