Help to add converter to attached lisp

[arshadmirza786]

It is a very good lisp by http://esurveying.net/land-survey/free-lisps

I need a modification to add a unit convertor with option Y/N 

When select (m) than all so convert to (ft) and when select (ft) than to (m)

"62_Mark_Chain"

[marko_ribar · Accepted solution]

If you haven't noticed that file is *.fas - protected lisp file and can't be modified... You should study what it does and create replication of routine in *.lsp form where you could do your desired intervention with converting (ft) to (m) and opposite (m) to (ft)...