Definintion of ``proto::context::null_context<>``, an evaluation context for `proto::eval()` that simply evaluates each child expression, doesn't combine the results at all, and returns void.

void
void `Expr &` `Context &`

For `N` in `[0,Expr arity)`, evaluate:

    proto::eval(proto::child_c<N>(expr), context)

An evaluation context for proto::eval() that simply evaluates each child expression, doesn't combine the results at all, and returns void.
null_context const
proto::context::null_eval
< Expr, ThisContext >