If Atom is instantiated, converts it to its integer form Integer.
Atom may contain only digits posibly preceded by a + or a -.
Success: integer_atom(1989,X). (gives X = '1989'). integer_atom(X,'+12'). (gives X = 12). integer_atom(-7,X). (gives X = '-7'). integer_atom(X,'-7'). (gives X = -7). integer_atom(N,'1234'). (gives N = 1234). Fail: integer_atom(I,'- 15'). integer_atom(I,' +15'). integer_atom(I,'2 '). integer_atom(1234,'Abcd'). integer_atom(222,'123'). integer_atom(x,'+12'). Error: integer_atom(A,B). (Error 4). integer_atom(124.5,X). (Error 5). integer_atom(N,1234). (Error 5).