Analysis
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v4 uses adaptive LL(*) or ALL(*). It is pronounced "all star". 
Parsing
- Left recursion elimination and expression grammar simplification [ANTLR4:DONE]
- Scannerless parsing
- Error alternatives
- Maybe ambiguous alt parsing, yielding parse forest
- Improve tree pattern matching, rewrites; e.g., <"0":primary>
- syntactic predicates ala PEGs; needed in token rules. ~ and ! are taken at the moment. perhaps the negate operator '-LetterOrDigit'; might as well at the positive version also: &LetterOrDigit.
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