add prune_with_map_projection

[hannes14]

@ederc

[ederc]

@RafaelDavidMohr Could you please have a look if these changes include the information you need?

[fingolfin]

I don't understand this docstring: There is one argument id; it is not stated how it related to R. And then the next line, is that transformation matrix meant to be another return value?

How about this (assuming I guessed the meaning right, which I probably didn't):

Suggested change

	Returns the module R minimally embedded in a free module such that the
	corresponding factor modules are isomorphic,
	Return a module `R`, a transformation matrix `M` and a projection map `p`
	satisfying the following properties: `R` is minimally embedded in a free
	module `parent(R)` such that the quotient `parent(R)/R` is isomorphic to `id`,
	with `M` describing this isomorphism, and `p` mapping `gen(id, i)` to
	`gen(id, p(i))`

I am still rather confused by the usage of gen(i) (what is i? An index ranging over 1 .. ngens(id) maybe?!?), and why p[i] -- is p a vector, or a function, or what?

[RafaelDavidMohr]

If @fingolfin's documentation update above is correct, then yes, this is what I need. Essentially I just need the map parent(R)/R -> id