Multiple instances of same option

It can be useful to allow users to provide options more than once. For example, the Guix Workflow Language accepts the "--input" option more than once. All values provided will be collated.

E.g.

guix workflow run --input=foo --input=bar --input=baz

This is not possible with Guile Config, because only the last option remains accessible (so (option-ref opts 'input) yields "baz").

It would be nice if an optional merge strategy could be provided. The default is to replace values. A merge strategy could be provided as a procedure, e.g.:

(lambda (elem previous)
  (if (pair? previous)
    (cons elem previous)
    (list elem previous))

What do you think?