Key dependent operation

[bh11]

add setter and tester operations for KeyDependentOperations

This should make it easier to handle the problems raised in #412 and #511.

In the long run, it might be a good idea to "undocument" the Computed… attribute itself, and maybe replace it by a dictionary object.

This PR also fixes a potential problem when computing a value, the Computed… attribute gets changed (because the called method computed other attribute values on the way, etc).

[bh11]

P.S. I might also add a binary search, somehow hurts to see a linear search in a sorted list.

[markuspf]

This seems like a prime application for the caching framework that @fingolfin was suggesting (also in view to HPC-GAP merge)

[hungaborhorvath]

Ah, so the syntax would be

HasSylowSubgroup(G, 2); 

and

SetSylowSubgroup(G, 2, subgroup);

Is that right?

[hungaborhorvath]

BTW, I find these as excellent and very useful functions, thank you very much for the effort.

Have not tested it, yet, though.

[bh11]

@hungaborhorvath Yes, that's the syntax. There's an example in the manual section.

[fingolfin]

This is a very nice idea. I also agree with @bh11's remarks on the Computed... attributes. As to the binary search: This is implemented by PositionSorted resp. PositionSet, perhaps they can be used (or perhaps they are already? I have not yet looked at the patch here itself, sorry).

UPDATE: I just noticed that there IS already an example (and thus test case) in the manual, so please disregard my bogus prior comment