support for built-in -opt style options for proc

I want to create a couple of procs with these usages:

dialog::prepare ?-parent .? ?-modal true? window

I can't use the parse_args package because I don't know how to install it
(and it has no INSTALL doc)

I can see from the wiki that there seems to be a lot of different pure Tcl
solutions, so I don't know which one to choose.

I'm using Tcl/Tk 9.0b2.