topComponents -- compute top dimensional component

The method used is that of Eisenbud-Huneke-Vasconcelos, in their 1993 Inventiones Mathematicae paper.

If M is a module in a polynomial ring R, then the implementations of topComponents and removeLowestDimension are based on the following observations:

• codim Ext^d(M,R) ≥d for all d
• If P is an associated prime of M of codimension d := codim P > codim M, then codim Ext^d(M,R) = d and the annihilator of Ext^d(M,R) is contained in P
• If codim Ext^d(M,R) = d, then there really is an associated prime of codimension d.
• If M is R/I, then topComponents(I) = ann Ext^c(R/I,R), where c = codim I