The calling-standard macros use the concept of a single, current,
    active routine. A routine is a programming entity that is
    associated with a procedure descriptor that may be called, or
    is a main routine specified as the transfer address of a linked
    image.

    Only one routine can be active or current at any given time
    during assembly. If more than one routine is defined in a single
    assembler source file, all items associated with the current
    routine, that is, within the lexical scope of the routine, must
    be completed before making a different routine current. The
    lexical scope of one routine cannot overlap the lexical scope
    of another routine.

    A routine becomes current or comes into scope by invoking the
    $ROUTINE macro with the appropriate arguments. $ROUTINE marks the
    beginning of the lexical scope of a routine. The complementary
    macro, $END_ROUTINE, marks the end of the current routine's
    lexical scope.