@property<EOL><INDENT>def ref_n(self):<DEDENT>
try:<EOL><INDENT>return int(self.ref)<EOL><DEDENT>except ValueError:<EOL><INDENT>return len(self.ref) if self.ref else None<EOL><DEDENT>
returns an integer, either from the `ref` instance variable if it's a number, or the length of
ref if it's a string, or None otherwise

>>> NARefAlt("ACGT").ref_n
4
>>> NARefAlt("7").ref_n
7
>>> NARefAlt(7).ref_n
7