program_options Can one have options with optional values?

[nobody]

Hello,

I was wondering if it is possible to have an option for
which supplying an argument is optional.  For example,
suppose I have an executable called myprog and an
option "o" that does something.  Is it possible to
allow both of the following command line syntaxes?

myprog --o arg
myprog --o

Where in the second case I can programatically supply
some default value?

So the first would instruct the program to take the
action associated with the "o" option using the
supplied arg and the second would instruct it to take
that action using the default argument value.

I have been fooling with the default_value and implicit
methods but neither seems to do what I want. 

Thanks,
John Eddy
johneddy@eng.buffalo.edu

[Vladimir Prus]

Dup of #342.