It is certain that Jeff dropped the apple. It is probably the case that Bernhard is yellow. We doubt that Jessica is a mouse. It is certain that if either 'Jessica is a mouse' or 'Bernhard is yellow' but not both then Greg is a swan. There is a very good chance that if either 'Bernhard is yellow' or 'Jeff dropped the apple' but not both then Brian is a frog. There is almost no chance that if 'Jessica is a mouse' or 'Jeff dropped the apple' or both then Julius is a swan.
It is improbable that 'Julius is a swan' or 'Greg is a swan' or both.
There is a better than even chance that 'Julius is a swan' or 'Greg is a swan' or both.
It is improbable that 'Julius is a swan' or 'Greg is a swan' 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
  1.0::factA.
  0.7::factB.
  0.2::factC.
  %hop1
  1.0::factX:-xor(factC,factB).
  0.8::factY:-xor(factB,factA).
  0.02::factZ:-or(factC,factA).

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

  query(conclusion).
  
'Julius is a swan' or 'Greg is a swan' or both.