There is a very good chance that Lily is white. There is a very good chance that Gertrude is a sheep. There is a very good chance that Emily is a mouse. We believe that if 'Emily is a mouse and Lily is white' then Julius is a swan. It is highly unlikely that if 'Emily is a mouse and Lily is white' then John took the apple. It is highly likely that if 'Gertrude is a sheep' or 'Lily is white' or both then Mary went to the bedroom.
It is certain that 'Julius is a swan' or 'John took the apple' or both.
Chances are about even that 'Julius is a swan' or 'John took the apple' or both.
It is certain that 'Julius is a swan' or 'John took the apple' 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
  0.8::factA.
  0.8::factB.
  0.8::factC.
  %hop1
  0.75::factX:-and(factC,factA).
  0.05::factY:-and(factC,factA).
  0.9::factZ:-or(factB,factA).

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

  query(conclusion).
  
'Julius is a swan' or 'John took the apple' or both.