underlying sequence, and change the way that the error handler is invoked, so
that it detects token iterators, and passes iterators into the underlying
range to the error handler, instead of the token iterators.
See #202.
parser_interface. token_spec is now a variable template that generates a
parser_interface wrapping a token_parser, which parameterized on the
token_spec_t. This way, a single token_spec use can be used to specify how to
lex, and how to parse.
See #202.
