10.1 Statement Modifiers
  Statement modifiers are control structures that operate
  on a single statement. Statement modifiers let you execute
  a statement conditionally or create a loop. The following are
  BASIC statement modifiers:

        IF
        UNLESS
        FOR
        UNTIL
        WHILE

  A statement modifier affects only the statement immediately
  preceding it. You can modify only executable statements;
  declarative statements cannot be modified.