How to filter for issues without an "implements" link

[Frank Schophuizen]

We have defined an implementation link type "is implemented by" / "implements", i.e. a parent is implemented by its children and a child implements a parent. I need to find all issues without a "implements" link (i.e. they don't implement a parent issue), but this issues may have their own "is implemented by" link. How to do that?

We have install ScriptRunner, so we have additional query functions like "hasLinkType". The following filter finds all issues without an implementation link.

issueFunction not in hasLinkType(Implementation)

But this filters out issues with a "implements" link as wel as issues with a "is implemented by" link. So those as issues without a parent and without children. But we want to find only the issues without a parent, i.e. without the "implements" links; they may have a "is implemented by" link.

[Frank Schophuizen]

I have tried something similar with Epic links: filter on issues outside the SYS project without an Epic link to an issue of the SYS project. issueFunction not in issueFieldMatch("project != SYS", "Epic Link", "^SYS-") (in fact, this example is a simplified part of an actual filter)