Add iteration to user actions

Hi,
AFAIK, it's not currently possible to iterate (à la for f in *.md, in bash for instance), in a user action, over a set of file names matched by a glob. Considering how it's frequent to resort to this in scripts, I would argue it's useful. Of course, a counter-argument is to avoid user actions to become a full-featured shell...
Use case: iterate over files to perform a given action, e.g. running expect (ocaml-mdx) tests; tentative syntax:

(alias
  (name regression)
  (deps 
    (package mypackage) 
    (:mds (glob_files *.md)) 
  )
  (action (foreach %{f} %{mds}; <-- iterate one by one over files matching the glob above
       (progn     
           (run ocaml-mdx test %{f})
           (diff? %{f} %{f}.corrected)))))

The issue names no repository files or tests; start by tracing the user-action handling and the proposed glob_files/foreach syntax. Clarify iteration semantics and use the ocaml-mdx example as the acceptance case, with tests covering matched files and the resulting per-file actions.