optional_value : value
                      | empty

def p_optional_value(t):
    """optional_value : value
                      | empty"""
    # return value or None.
    t[0] = t[1]
    # Note this must be unsigned
    value = t[0]
    if value is None or value[0].isdigit():
        return
    msg = ''
    if value[0] == '-':
        msg = "Can't use negative index %s" % value
    elif value not in name_dict:
        msg = "Can't derefence index %s" % value
    else:
        data = name_dict[value]
        if data.type != 'const':
            msg = "Can't use non-constant %s %s as index" % (data.type, value)
        elif not data.positive:
            msg = "Can't use negative index %s" % value
    if msg:
        global error_occurred
        error_occurred = True
        print(u"ERROR - {0:s} near line {1:d}".format(msg, t.lineno(1)))