It is improbable that Brian is a lion. Chances are about even that Bernhard is a frog. Chances are about even that Mary went to the office. We doubt that if 'Mary went to the office' or 'Brian is a lion' or both then John discarded the apple. There is a very good chance that if either 'Brian is a lion' or 'Mary went to the office' but not both then Sandra took the football. Chances are about even that if 'Brian is a lion' or 'Bernhard is a frog' or both then Greg is a rhino.
There is a better than even chance that 'Greg is a rhino' or 'Sandra took the football' or both.
There is a better than even chance that 'Greg is a rhino' or 'Sandra took the football' or both.
It is highly unlikely that 'Greg is a rhino' or 'Sandra took 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.
  0.5::factB.
  0.5::factC.
  %hop1
  0.2::factX:-or(factC,factA).
  0.8::factY:-xor(factA,factC).
  0.5::factZ:-or(factA,factB).

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

  query(conclusion).
  
'Greg is a rhino' or 'Sandra took the football' or both.