MergeError if tag as list inherits tag as a string from the upper node

[smyle10]

tmt version: 1.35.0

Given a main.fmf:

test: TEST_ITEMS=${TMT_TEST_NAME##*/} bash ./runtest.sh
tag: foobar
/test_keep_iproute_routes:
    tag+:
      - customer
      - unmanaged

Running tmt test ls got:

MergeError: Key 'tag' in /networking/nmstate/anyhost/route/test_keep_iproute_routes (can only concatenate str (not "list") to str).

Though tmt hints the reason and I can change string "foobar" as a list (but consider a new tmt user may not know the inheritance feature in tmt) , as https://tmt.readthedocs.io/en/stable/spec/core.html#tag indicates that a tag can be string or list, hope such merge can be improved.

[LecrisUT]

This is an fmf issue. If you make the parent a list that would make it work. Maybe the parent could be converted to a list beforehand, but then it would make concatenation of strings impossible.

The full list of operators are here. Do you have an idea of an interface that could help here? Also related discussion here (the opposite case to yours)