It is certain that Mary went to the office. It is probably not the case that Emily is a wolf. It is probable that John dropped the apple. It is highly unlikely that if 'Mary went to the office' or 'John dropped the apple' or both then Greg is a swan. It is highly likely that if 'John dropped the apple and Mary went to the office' then Bill left the football. It is unlikely that if 'John dropped the apple' or 'Mary went to the office' or both then Brian is a rhino.
Chances are about even that 'Mary went to the office' or 'Greg is a swan' or both.
It is certain that 'Mary went to the office' or 'Greg is a swan' or both.
Chances are about even that 'Mary went to the office' 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.
  0.25::factB.
  0.7::factC.
  %hop1
  0.05::factX:-or(factA,factC).
  0.9::factY:-and(factC,factA).
  0.2::factZ:-or(factC,factA).

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

  query(conclusion).
  
'Mary went to the office' or 'Greg is a swan' or both.