dsl::context_counter "is_not", "is_not_zero" functions

[SC5Shout]

I am probably missing something, and there may be a better way to do this, but it'd be nice to add "is_not" and "is_not_zero" functions to the context counter. This would allow, for example, checking if a list of variants has at least one instance of a specific object.

eg.:

struct ProdA;
struct ProdB;

struct Production
{
	auto rule = [](){
		constexpr auto counter = dsl::context_counter<ProdA>;
		constexpr auto decl = (dsl::p<ProdA> >> counter.inc() | dsl::p<ProdB>);
		return counter.create() + dsl::curly_bracketed.list(decl) + dsl::must(counter.is_not_zero()).error<struct MissingProdA>;
	}();

	auto value = lexy::as_list<std::vector<std::variant<ProdA, ProdB>>>;
};

Adding this is as easy as:

• copy-paste struct _ctx_cis,
• rename to struct _ctx_cisnot
• and change in try_parse to return _ctx_counter<Id>::get(cb) != Value;

[foonathan]

Sure that's reasonable, do you want to do a PR? Maybe change _ctx_cis to something like:

template <typename Pred>
struct _ctx_cis;

and then expose an overload for counter.is<Predicate>().

[SC5Shout]

Sure, please check

[SC5Shout]

I see some tests failed, lemme fix them first

[foonathan]

This is added in #239.