Issue 3: For comprehensions should be type checked
Project Member Reported by adria...@gmail.com, Mar 26, 2007
 * Define a trait (re-use Iterable?) that should be implemented for for-comprehensions to be 
translated to map/flatMap/filter/foreach calls.

 * Split type checking of for-comprehensions in
  # translate as-is, without syntax transformation
  # check that targets implement proper type (Iterable)

 * Improve error messages.