It is improbable that Mary left the football. It is highly unlikely that Lily is green. It is improbable that Greg is a frog. There is little chance that if either 'Mary left the football' or 'Lily is green' but not both then Emily is a sheep. We doubt that if 'Mary left the football and Lily is green' then John picked up the milk. It is highly likely that if either 'Lily is green' or 'Greg is a frog' but not both then Brian is white.
It is unlikely that 'Greg is a frog' or 'Mary left the football' or both.
It is unlikely that 'Greg is a frog' or 'Mary left the football' or both.
We believe that 'Greg is a frog' or 'Mary left the football' 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.
  0.05::factB.
  0.1::factC.
  %hop1
  0.1::factX:-xor(factA,factB).
  0.2::factY:-and(factA,factB).
  0.9::factZ:-xor(factB,factC).

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

  query(conclusion).
  
'Greg is a frog' or 'Mary left the football' or both.