<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _listify(x):
 """Ensure x is iterable; if not then enclose it in a list and return it."""

    if isinstance(x, string_types):
        return [x]
    elif isinstance(x, collections.Iterable):
        return x
    else:
        return [x]