underscore should be used as a variable loop only iff its value is not used

otherwise, one should use a normal variable

CC: @tscrim @slel @kliem @orlitzky

Component: combinatorics

Author: Frédéric Chapoton

Branch/Commit: b34161e

Reviewer: Michael Orlitzky

Issue created by migration from https://trac.sagemath.org/ticket/32978