identifying readonly lines; filtering inserted text

["Marvin Lyndon" <marvin.lyndon@xxxxxxxxxxx>]

Dear fellow jed users,

I'm giving my first steps in slang/jed scripting, and would appreciate some 
help.
I am trying to provide for some basic "filtering"/"validation" when the user 
is updating some lines in a structured file. I know how to make a line 
readonly by calling set_line_readonly(1); is there a way to detect attempts 
to modify a line which is marked as readonly? My goal is to run a slang 
function each time the user tries to modify a readonly line.
I have noticed that a Runtime Error exception is emitted when I try to do 
so; but is there a way for me to install a handler/"catch" statement for 
those exceptions? Even if I succeed in doing so, how do I know that the 
Runtime Error exception was generated by an attempt to write to a readonly 
line? (No function equivalent to is_line_hidden() seems to exist...) I 
thought it would be trivial to use update_hook for this, but it only calls 
functions *after* the text insertion attempt has happened and so doesn't 
give me a chance to detect it.
Any guidance would be great.

Thanks so much

Marvin

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