Note
	Note that you can find everything that has been included and defined so far here.

Currently we use the plus_token for parsing "the" operator, which has to be +. We can define a new token for parsing the - symbol:

> using minus_token = token<lit_c<'-'>>;

We need to build a parser that accepts either a + or a - symbol. This can be implemented using one_of:

> #include <boost/metaparse/one_of.hpp>

one_of<plus_token, minus_token> is a parser which accepts either a + (using plus_token) or a - (using minus_token) symbol. The result of parsing is the result of the parser that succeeded.

	Note
	You can give any parser to one_of, therefore it is possible that more than one of them can parse the input. In those cases the order matters: one_of tries parsing the input with the parsers from left to right and the first one that succeeds, wins.

Using this, we can make our parser accept subtractions as well:

> using exp_parser12 = \
...> build_parser< \
...>   foldl_start_with_parser< \
...>     sequence<one_of<plus_token, minus_token>, int_token>, \
...>     int_token, \
...>     boost::mpl::quote2<sum_items> \
...>   > \
...> >;

copy-paste friendly version

It uses one_of<plus_token, minus_token> as the separator for the numbers. Let's try it out:

> exp_parser12::apply<BOOST_METAPARSE_STRING("1 + 2 - 3")>::type
mpl_::integral_c<int, 6>

The result is not correct. The reason for this is that sum_items, the function we summarise with ignores which operator was used and assumes that it is always +.