Logical Variables (I)

• Logical variables provide argument passing (to and from predicates)
• And more interesting stuff (we will see later)
• Logical variable assignment is monotonic

  ?- X = a.
  X = a

  ?- X = a, X = b.
  no
  (because X cannot be at the same time a and b)

• The constraint $=/2$ forces the variables to have the same value

  ?- X = Y, X = a.
  X = a, Y = a.

  ?- X = Y, pet(X), sound(Y, roars).
  no
  (why?)

  ?- X = Y, pet(X).
  X = spot, Y = spot
  X = barry, Y = barry