Daniel, I examined LinearFunctionQ and found:
- It handles expressions written with normal symbols x, y, ... correctly
- It does respect variables written as Indexed (say Indexed[x,1])
- It does respect variables written as Subscript(ed) (say Subscript[x,1])
Hence your function is perfect (BTW: Is the source code available?).
My code may be overloaded, since it outputs detailed messages to tell the user exactly what is wrong with which equation. For simplicity, I think I'll throw that out and use your function.
Thanks for that hint!
