If String is instantiated, converts it to an atom Atom.
Success: atom_string('Tom',"Tom"). atom_string(tom,X). (gives X="tom"). atom_string(X,"4"). (gives X='4'). Fail: atom_string('jo',"joe"). Error: atom_string(X,Y). (Error 4). atom_string(4,"4"). (Error 5). atom_string(tom,'tom'). (Error 5).