There is almost no chance that Sandra dropped the milk. We believe that Greg is yellow. It is highly unlikely that Gertrude is a mouse. It is likely that if either 'Gertrude is a mouse' or 'Greg is yellow' but not both then Julius is green. It is highly likely that if 'Gertrude is a mouse and Greg is yellow' then Brian is a swan. It is highly unlikely that if either 'Greg is yellow' or 'Sandra dropped the milk' but not both then John got the milk.
It is improbable that 'Gertrude is a mouse' or 'John got the milk' or both.
It is improbable that 'Gertrude is a mouse' or 'John got the milk' or both.
It is probably the case that 'Gertrude is a mouse' or 'John got 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
  0.02::factA.
  0.75::factB.
  0.05::factC.
  %hop1
  0.7::factX:-xor(factC,factB).
  0.9::factY:-and(factC,factB).
  0.05::factZ:-xor(factB,factA).

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

  query(conclusion).
  
'Gertrude is a mouse' or 'John got the milk' or both.