It is highly unlikely that Mary grabbed the milk. There is a very good chance that Bill moved to the office. We believe that Greg is a frog. It is highly unlikely that if 'Mary grabbed the milk and Greg is a frog' then Emily is a sheep. It is unlikely that if either 'Mary grabbed the milk' or 'Greg is a frog' but not both then Julius is a frog. It is probably the case that if either 'Greg is a frog' or 'Bill moved to the office' but not both then John dropped the apple.
We believe that 'Greg is a frog' or 'Emily is a sheep' or both.
We believe that 'Greg is a frog' or 'Emily is a sheep' or both.
There is almost no chance that 'Greg is a frog' or 'Emily is a sheep' 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
  0.05::factA.
  0.8::factB.
  0.75::factC.
  %hop1
  0.05::factX:-and(factA,factC).
  0.2::factY:-xor(factA,factC).
  0.7::factZ:-xor(factC,factB).

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

  query(conclusion).
  
'Greg is a frog' or 'Emily is a sheep' or both.