Fix grammar and allow plural "(s)" #37
                
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Removes an implicit match for "" (nothing) because I left the first line after the ":" empty. Obviously not what is intended! So while it would be more readable to be able to wrap everything inside a rule with an indent, it doesn't seem to be supported currently [1].
Also, allow appending
(s)to indicate plural in expressions likelist of int(s). A negative lookbehind regex disallows a space directly in front of(s).Other changes improve on naming, structure and tests.
[1] www.github.com/lark-parser/lark/issues/155