There is little chance that Greg is white. We doubt that Bernhard is a swan. There is a better than even chance that Mary dropped the apple. It is probably not the case that if 'Greg is white and Mary dropped the apple' then Jason is tired. It is probably not the case that if either 'Mary dropped the apple' or 'Bernhard is a swan' but not both then Winona is a sheep. Chances are about even that if 'Mary dropped the apple and Bernhard is a swan' then John left the apple.
It is highly unlikely that 'John left the apple' or 'Jason is tired' or both.
It is highly unlikely that 'John left the apple' or 'Jason is tired' or both.
There is a better than even chance that 'John left the apple' or 'Jason is tired' 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.
  0.2::factB.
  0.6::factC.
  %hop1
  0.25::factX:-and(factA,factC).
  0.25::factY:-xor(factC,factB).
  0.5::factZ:-and(factC,factB).

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

  query(conclusion).
  
'John left the apple' or 'Jason is tired' or both.