Renamed union

[Adda0]

This PR resolves the issue with naming the operation union from #172. This PR is a rebase and update of #354.

There are currently two versions of the union operation:

1. nondeterministic union (the classic union), called union_nondet(), and
2. union using product construction (performing product construction and adding final states whenever either automaton has the corresponding state in the product state final), called union_product().

The in-place variant of union_nondet() is called unite_nondet_with(), as per the suggestion in #277 (comment).

Another suggestion for the union_nondet() was lang_union().

[jurajsic]

I think there should still be some main simply-named union function (uni, unionn, unite, or whatever), because all other operations have such functions. It can even have parameters, like other functions (one now could be preserve_determinism), but maybe we do not want that.

[Adda0]

What would the function do, however? I think that the most basic "union" operation is the current union_nondet(). Therefore, the function would just call the union_nondet(), and if the parameter preserve_determinism was set, the function would just call union_product(). And the purpose of this whole PR was to remove the general uni() function. If we were to return it back, the naming problem would be here again. I personally am all for unite(), for example, but I am not sure what the purpose of such a function would be if the only thing the method does is calling other methods without any reasonable modifications.

[jurajsic]

Ah, so the point of this PR was to remove the general function. Then yes, this solution is ok.

The purpose would be for the user to not have to think what function to use for union, if we want for him to always think and decide what to use, then this is ok. It would be very similar to how is_lang_empty() is implemented, by default it just calls one of the specialized emptiness test functions.

[Adda0]

Not necessarily to remove the general function, but to remove the naming problem for union() function which cannot be called union. We could maybe agree on adding a general function called unite(), or lang_union(), for example, which would call these two functions.

[Adda0]

@kilohsakul Any preferences?

[jurajsic]

Same as in previous comment, keep some simply-named union function.

[Adda0]

I pushed the agreed upon changes. We will resolve whether to add a general union function in another issue and PR. For now, I will merge the changes, so I can create PRs for other dependent changes.