.Q.trp and bt handling - kdb

Question

I saw (in .Q.fpn) the following technique to parse and show the bt object passed to .Q.trp function:
q)f:{x+1}
q).Q.trp[f;`a;{'(x;y)}]
'type
[4] f:{x+1}
^
[3] (.Q.trp)
[2] .Q.trp[f;`a;{'(x;y)}]
^
[0] .Q.trp[f;`a;{'(x;y)}]
^
'(x;y) seems like an exception building construction, - but Kx documentation says that there are only two ways of exception building: from symbol and from string. It is looks like we can built an exception from a list of (symbol; bt object).
So what the construction '(x;y) stands for?
Can we build something different than exception with '(x;y)?

Answer 1

My guess is that this is a specific signal recently allowed along with the addition of the .Q.trp/.Q.bt functionality. It looks like it works only for (symbol;bt object) or (string;bt object), anything else is unrecognized.
q).Q.trp[{1+x};`a;{'(x;y;1)}]
'stype
The output can be stored if returned without the signal:
q)r:.Q.trp[{1+x};`a;{(x;y)}]
and this type of signal seems to work in any context, not just within .Q.trp:
q)'("other";last r)
'other
[2] {1+x}
^
[1] (.Q.trp)
[0] r:.Q.trp[{1+x};`a;{(x;y)}]
I suspect the last r has a very specific format/shape that one could fabricate but it seems like an unnecessary use-case.
Bonus oddities:
This works:
q)'("other";())
'other
[0] '("other";())
^
but other things I've tried show up weird errors:
q)'("other";(();()))
pl0
pl0
q)
q)'("other";"abc")
srr