[x3-devel][design]add is_default_parse_rule=true flag to rule_definition::parse

In the parse_rhs_main function here:

https://github.com/boostorg/spirit/blob/develop/include/boost/spirit/home/x3/nonterminal/detail/rule.hpp#L204

there's a flag, is_default_parse_rule, calculated earlier in the 
function.  Why couldn't this flag be passed as an argument instead?
Then, here:

https://github.com/boostorg/spirit/blob/develop/include/boost/spirit/home/x3/nonterminal/rule.hpp#L162

it would be passed as false, and the default value of true could be
added to the rule_definition::parse function here:

https://github.com/boostorg/spirit/blob/develop/include/boost/spirit/home/x3/nonterminal/rule.hpp#L58

This would be clearer because there would be no need for the calculation
in the earlier part of parse_rhs_main, and, since it's being passed
as false when called from the BOOST_SPIRIT_DEFINE macro, the correctness
of the value would be obvious in this case.

Is there any downside to this?

-regards,
Larry


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