Defines macros for third-person singular pronouns (\E, \Em,
\Eir, \Eirs), which expand differently according to a
masculine/feminine switch. (If the switch is 'masculine', they
would expand to 'he', 'him', 'his' and 'his'; if 'feminine',
they would expand to 'she', 'her', 'her' and 'hers'. Apart from
the pronouns, one can define 'word pairs', such as
mother/father, daughter/son, and so on. Gender may be defined
once per document, as an environment, or may be flipped on the
fly.
