It is probably the case that Emily is a mouse. It is probable that Greg is yellow. There is a very good chance that Sandra dropped the milk. It is probably the case that if 'Sandra dropped the milk and Greg is yellow' then Brian is a swan. It is highly unlikely that if either 'Sandra dropped the milk' or 'Emily is a mouse' but not both then Julius is gray. Chances are slight that if 'Greg is yellow' or 'Emily is a mouse' or both then Sumit is hungry.
It is probably not the case that either 'Greg is yellow' or 'Brian is a swan' but not both.
It is probably not the case that either 'Greg is yellow' or 'Brian is a swan' but not both.
We believe that either 'Greg is yellow' or 'Brian is a swan' 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
  0.7::factA.
  0.7::factB.
  0.8::factC.
  %hop1
  0.7::factX:-and(factC,factB).
  0.05::factY:-xor(factC,factA).
  0.1::factZ:-or(factB,factA).

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

  query(conclusion).
  
Either 'Greg is yellow' or 'Brian is a swan' but not both.