Arg.spec and option types

[Nicolas barnier <barnier@recherche.enac.fr>]

Hi list,

I find the recent addition of the Set_{string|int|float} constructors
of type Arg.spec quite useful, as it spares a lot of repeated code
for processing command line arguments. However, for optional arguments,
I often feel the need (and write the code) to handle option types,
which can elegantly be processed to trigger optional behaviours of a
program.

I think it would therefore be quite useful to add 
Set_option_{string|int|float}
constructors which would expect {string|int|float} option ref values as
arguments (usually initialized with None). Do others think the same or use
some other workaround to obtain this behaviour ?

-- Nicolas