Return a reference to a rule.
Defined in header
template< class Rule> constexpr implementation-defined ref( Rule const& r);
This function returns a rule which references the specified object. This
is used to reduce the number of bytes of storage (
required by a combinator when it stores a copy of the object.
Ownership of the object is not transferred; the caller is responsible for
ensuring the lifetime of the object is extended until it is no longer referenced.
For best results,
only be used with compile-time constants.
The rule to use