start_production parameter in parser constructor

[SupraSummus]

For me it is misleading. I think there are two problems with it:

• it allows just for single production, whereas it should accept a start symbol (which can expand to multiple productions)
• it is a part of parser constructor, whereas (in formal grammar definition) start symbol/production is a part of grammar specification.

So I propose moving start symbol/production parameter to Grammar constructor and forbidding to specify it in Parser constructor.

There is start_symbol parameter for Grammar constructor, but it seems unused. (I may be unfamiliar with code and missed something.) The only place it is used is around parglare/grammar.py:861 where based on it AUGSYMBOL rule is inserted as a first production. But then parser uses production of index 1 as a default starting production, so it makes no sense to me.

[igordejanovic]

Implementation around start_production indeed needs a cleanup. It is a left-over that was not reworked completely. The initial idea for specifying start_production on the parser was to enable user to instantiate a parser for the part of the language, but I haven't had that use-case yet so not sure that we should support it at all. After introduction of table serialization it only complicates matter.

[SupraSummus]

If the use case for instantiating parser for part of the language is viable, then it can be satisfied with start_symbol in Grammar constructor. User could take existing grammar and change start_symbol. But it needs to be implemented before ;)