ly:add-option behaviour

[Urs Liska]

I'm wondering about the behaviour of ly:add-option

If I have the following file:

\version "2.19.47"

#(ly:add-option 'humdrum-file #f "Set to the file name of a Humdrum file")

#(display (ly:get-option 'humdrum-file))

and invoke LilyPond with the command line option -dmy-option=my-value

The display is #f or whatever I set as default value in ly:add-option.

If I comment out the ly:add-option line I get the correct value
displayed but also a warning "no such internal option: my-option".

I thought I could use that line to suppress the warning and preset the
option with a default value that it will have when the option isn't
passed on the command line. But I have the impression that ly:add-option
*overwrites* any value passed through the command line.

What am I missing?

Best
Urs