[x3_devel] why not static ... def_ in specialized parse_rule?

The code here:

https://github.com/boostorg/spirit/blob/x3-devel/include/boost/spirit/home/x3/nonterminal/rule.hpp#L167

recalculates the same const value each time the function is entered.
Why not make this static to avoid the recalculation?

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