Refactor `generic_lines`

The following discussion from !1428 (merged) should be addressed:

• @sstahl started a discussion: (+1 comment)

  Maybe we could move the line to the generic_lines folder and include it in the tests of these. But not sure.

mentioned in merge request !1428 (merged)

mentioned in issue #407

Done with 85601143

closed

marked the checklist item @sstahl started a discussion: (+1 comment) as completed