def check_len_in(self, min_len, max_len, item):
        """Checks that the length of item is in range(min_len, max_len+1)."""
        if max_len is None:
            if min_len:
                self.add_check("_coconut.len(" + item + ") >= " + str(min_len))
        elif min_len == max_len:
            self.add_check("_coconut.len(" + item + ") == " + str(min_len))
        elif not min_len:
            self.add_check("_coconut.len(" + item + ") <= " + str(max_len))
        else:
            self.add_check(str(min_len) + " <= _coconut.len(" + item + ") <= " + str(max_len))
Checks that the length of item is in range(min_len, max_len+1).