It is impossible that John took the football. Chances are slight that Jessica is a sheep. It is impossible that Julius is a swan. Chances are about even that if 'Jessica is a sheep' or 'John took the football' or both then Bernhard is a lion. Chances are about even that if either 'John took the football' or 'Julius is a swan' but not both then Mary grabbed the milk. It is probable that if 'John took the football' or 'Jessica is a sheep' or both then Greg is a rhino.
It is highly unlikely that either 'Greg is a rhino' or 'Bernhard is a lion' but not both.
It is highly unlikely that either 'Greg is a rhino' or 'Bernhard is a lion' but not both.
It is certain that either 'Greg is a rhino' or 'Bernhard is a lion' but not 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
  0.0::factA.
  0.1::factB.
  0.0::factC.
  %hop1
  0.5::factX:-or(factB,factA).
  0.5::factY:-xor(factA,factC).
  0.7::factZ:-or(factA,factB).

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

  query(conclusion).
  
Either 'Greg is a rhino' or 'Bernhard is a lion' but not both.