It is highly unlikely that Julius is a swan. It is probably the case that Sandra left the milk. It is highly unlikely that Gertrude is a cat. It is highly unlikely that if 'Julius is a swan and Gertrude is a cat' then Greg is yellow. It is probably not the case that if 'Gertrude is a cat' or 'Sandra left the milk' or both then Lily is a lion. It is probably not the case that if either 'Julius is a swan' or 'Gertrude is a cat' but not both then Brian is gray.
There is a better than even chance that 'Greg is yellow and Brian is gray'.
It is impossible that 'Greg is yellow and Brian is gray'.
There is a better than even chance that 'Greg is yellow and Brian is gray'.

  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.
  0.7::factB.
  0.05::factC.
  %hop1
  0.05::factX:-and(factA,factC).
  0.25::factY:-or(factC,factB).
  0.25::factZ:-xor(factA,factC).

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

  query(conclusion).
  
'Greg is yellow and Brian is gray'.