Add BNF generation

src/bnf.rs

@@ -0,0 +1 @@
+pub struct Bnf {}

src/lib.rs

@@ -1,11 +1,17 @@
 #![feature(assert_matches)]
 #![allow(dead_code)] //TODO eventually turn this off
+mod bnf;
+
+use bnf::Bnf;
 use std::rc::Rc;
 
 type ParseResult<I, O, E> = Result<(O, I), E>;
 
 trait Parser<I, O, E> {
     fn parse(&self, input: I) -> ParseResult<I, O, E>;
+    fn bnf(&self) -> Option<Bnf> {
+        None
+    }
 
     fn map<'a, F, O2>(self, map_fn: F) -> BoxedParser<'a, I, O2, E>
     where