* include/vaucanson/algebra/concept/letter.hh: Add a new trait.
	* include/vaucanson/algebra/implementation/letter/char_letter.hxx,
	* include/vaucanson/algebra/implementation/letter/int_letter.hxx:
	* Specialization of default_epsilon.
	* include/vaucanson/algebra/implementation/series/polynoms.hxx,
	* include/vaucanson/algebra/implementation/series/rat/dump_visitor.hxx:
	* Be generic.
	* src/demos/one_rule_rewriting/check_rw_composition.cc: Add missing
	* header and refactorize some code.
	* taf-kit/src/b_commands.cc,
	* taf-kit/src/commands_macros.hh,
	* taf-kit/src/common.hh,
	* taf-kit/src/fmp_commands.cc,
	* taf-kit/src/main.cc,
	* taf-kit/src/parser_options.hh,
	* taf-kit/src/parser_options.hxx,
	* taf-kit/src/z_commands.cc: Use the new letter trait
	* (default_epsilon) to handle epsilon more generically.