code: def _concat(prefix, list, suffix, env, f=lambda x: x, target=None, source=None):
      
      if not list:
          return list
  
      l = f(SCons.PathList.PathList(list).subst_path(env, target, source))
      if l is not None:
          list = l
  
      return _concat_ixes(prefix, list, suffix, env)
docs: Creates a new list from 'list' by first interpolating each element
      in the list using the 'env' dictionary and then calling f on the
      list, and finally calling _concat_ixes to concatenate 'prefix' and
      'suffix' onto each element of the list.