Implement DeadNTermTagsElim transform with tests - #15
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#9
Implemented DeadNTermTagsElim.
The transform computes a conservative set of possible non-terminal tags with a fixed-point analysis and removes tag rules whose conditions cannot be satisfied.
It handles unless by checking satisfiability of cond AND NOT unless.
It assumes valid RichGrammar; invalid tag-condition bindings should be caught by ToRichGrammar/InternalCheck.
Added regression tests for impossible argument tags, fixed-point propagation, unless, multiple tags, and split-level-generated dead tags.