Extended Backus-Naur Form definition language
Any variation on the basic Backus-Naur Form
(BNF) meta-syntax notation with (some of) the following additional constructs: square brackets
"[..]" surrounding optional items, suffix "*" for Kleene closure
(a sequence of zero or more of an item), suffix "+" for one or more of an item, curly brackets enclosing a list of alternatives, and super/subscripts indicating between n and m occurrences.
All these constructs can be expressed in plain BNF using extra productions
and have been added for readability and succinctness.