def is_contradictory(self, other):
        """
        Can these two strings coexist ?
        """
        other = StringCell.coerce(other)

        if self.value is None or other.value is None:
            # None = empty, and won't contradict anything
            return False

        def sequence_in(s1, s2):
            """Does `s1` appear in sequence in `s2`?"""
            return bool(re.search(".*".join(s1), s2))

        return not sequence_in(self.value, other.value) and \
            not sequence_in(other.value, self.value)
Can these two strings coexist ?