Implement Sum.findrecur (with Sister Celine's algorithm)

fix #5511

- [ ] better name?
- [ ] Not sure if this should be a Sum method.  An alternative: some helper function.
> How?  Lower bounds can have some value here (i.e. at the line 462),
> but not upper.

At least one could break the loop there. This would avoid run-away
loops in case anything within the body goes wrong.

The global structure is: try with this i, j and if it does
not succeed then increase the values. It should be possible
to stop once we reach the upper bound anyway.