Remove the custom expeted
Implement a lexer for EBNF grammars
Simplify custom values in SequenceParser
Initiate a parser for Ebnf grammars
Implement definitions-list 4.4
Implement single-definition 4.5
Simplify
Implement syntactic-factor 4.8
Re-order
Implement 4.10 to 4.16
Fix naming
Don't yield Comment tokens
Implement 4.6 and 4.7
Implement 4.21 Empty
Add two tests