Writing IDL and other definition files

[Jeff L]

I am looking at writing a piece of IDL and a header file I am wrestling 
with how to do it without infringing copyright.  IDL looks particularly 
hard to write from first principles and is probably hard to write in a 
different way as the functions being implemented are defined and require 
IDL similar code.  Is there any advice as to what type of code can be 
copied and what constitutes a rewrite to avoid copyright?

Jeff