Plans regarding notions associated to commutativity in general.
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Detailed Description
Plans regarding notions associated to commutativity in general.
 Todo:
 Complexity of finding centralising elements

We have already centraliser_grd, bicentraliser_grd and centre.

Given an implicit representation of a groupoid together with the guarantee that "short" representations always exists (a "blackbox" groupoid), the problem of finding a centraliser of a subset is in NP.

Is it NPcomplete?

In a monoid the problem of finding just one centralising element needs to exclude the neutral element, and for centralising just a single element we also need to exclude the subgroupoid generated by this single element. So perhaps the general problem is, given some subgroupoid, finding a centralising element not in this subgroupoid.

Does there exist at this level of generality any other algorithm for finding centralising elements (one/all) other running through all elements?

Also the task of sampling needs consideration.

DONE Likely these functions should be renamed "centraliser_grd", "bicentraliser_grd" and "centre".
Definition in file Commutativity.hpp.