[S] Change in osmo-ttcn3-hacks[master]: library: add f_gsm_fn_{sum,sub,diff}()

Attention is currently required from: fixeria.

osmith has posted comments on this change by fixeria. ( https://gerrit.osmocom.org/c/osmo-ttcn3-hacks/+/42915?usp=email )

Change subject: library: add f_gsm_fn_{sum,sub,diff}() ......................................................................

Patch Set 2:

(1 comment)

File library/GSM_Types.ttcn:

https://gerrit.osmocom.org/c/osmo-ttcn3-hacks/+/42915/comment/8ca8291f_8989b... : PS2, Line 450: function f_gsm_fn_diff(GsmFrameNumber a, GsmFrameNumber b) return GsmFrameNumber { f_gsm_fn_diff is unused, even in https://gerrit.osmocom.org/c/osmo-ttcn3-hacks/+/42916/2. would it maybe make more sense to only introduce it later when it actually gets used?