Do'h! I missed the part that says that a string can start w/ '--'. Mea culpa.

I am aware of use of [:=] to make the option take a an optional argument or not. As presented in the code, i used '=' to require a value for the specified option.

I also noted, though very indirectly, in OP that "pass_through" makes the process interesting when i wrote "other options are passed as is". My frustation resulted after trying various configuration options.

All in all, thanks for the clue. I will try other things.