Add recursion for Sequences.

[vletard]

The recent addition of Tuples introduced recursion in the analogical verification and solving processes. Sequences can also be considered as recursive items (for instance with Sequences of Tuples, or Sequences of Sequences).
Doing so comes with a very high risk of combinatorial explosion, so this feature must come with controls for limiting computation or even disabling it entirely if necessary.
An option for dealing with recursive Sequences is to consider a fifth Step along with AB, AC, CD, BD: recursion. This would simply attempt to consume A, B, C and D while verifying/solving the corresponding sub-equation.
Also, the notion of degree has to be smartly adapted to such a resursive Step.