k-regular sequences: subsequences, forward/backward difference, partial sums #21325
Description
Implement subsequences and forward/backward differences and partial sums.
See also meta ticket #21202
Depends on #21318
CC: @galipnik
Component: combinatorics
Author: Daniel Krenn, Clemens Heuberger
Branch/Commit: 7bc6722
Reviewer: Clemens Heuberger, Daniel Krenn
Issue created by migration from https://trac.sagemath.org/ticket/21325