Loop
The tp::Loop<T> type is also a list of T (like Plus and Star), but
it will keep skipping tokens when parsing T fails until the end of input.
This is useful if the root should be a list of items, and you may want to skip invalid tokens in between.
Production
Loop<T> => T Loop<T>
Deref
Just like Plus and Star, Loop<T> can be dereferenced to &Vec<T>.