<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _concat(prefix, list, suffix, env, f=lambda x: x, target=None, source=None):
 """ 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. """

    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)