Unification and instantiation

1. Any value can be unified with itself. (This is normally useful only as a test.)

   Example: mother(john) = mother(john).

2. A variable can be unified with another variable. The two variable names thereafter reference the same variable.

   Example: X = Y, X = 2, write(Y). /* Writes the value 2. */

3. A variable can be unified with any Prolog value; this is called instantiating the variable. A variable is fully instantiated if it is unified with a value that does not itself contain variables.

   Example: X = foo(bar, [1, 2, 3]). /* X is fully instantiated. */
   Example: Pa = husband(Ma). /* Pa is partially instantiated. */

4. Two different values can be unified if there are unifications for the constituent variables which make the values the same.

   Example: mother(mary, X) = mother(Y, father(Z)).
   [Also results in the unifications mary=Y and X=father(Z)].

5. It is legal to unify a variable with an expression containing itself; however, the resultant value cannot be printed, and must otherwise be handled with extreme care.

   Example: X = foo(X, Y).