def valid(self):
    if (not self.value):
        return True
    if ((not (len(self.value) == 4)) or (not self.value.islower())):
        return ("OpenType feature tag '%s' needs to be a four-letter lowercase tag." % self.value)
    return True
def valid(self):
    if (not self.value):
        return True
    if ((not (len(self.value) == 4)) or (not self.value.islower())):
        return ("OpenType feature tag '%s' needs to be a four-letter lowercase tag." % self.value)
    return True