compare of 2 existing methods to implement grammar recursion

The repository:
   https://github.com/boostorg/spirit/tree/develop
has toy.cpp here:
   */workbench/x3/toy/toy.cpp
which connects the rule with it's rule_definition using
the context; however, it's not clear from the code how
to connect multiple rule's withe their rule_definition's.

In contrast:

   */include/boost/spirit/home/x3/nonterminal/rule.hpp

uses the BOOST_SPIRIT_DEFINE macro to do this, as illustrated by:

   */example/x3/calc/calc1.cpp

The gist:

   https://gist.github.com/cppljevans/3634cc4e8ad4da81531608e6aa847de5

shows both methods except, in contrast to toy.cpp, it uses
make_defns to link multiple rule's with their definitions.

Also, the gist uses a macro, USE_PARSE_RULE, to switch between the
2 methods to ease comparison.  It also has *plenty* of debug
prints to help clarify how it works.

It helped me understand the x3 design a bit better and
hopefully the above gist will help others do the same.

HTH.

-regards,
Larry



------------------------------------------------------------------------------