<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _set_fallback(elements, src_field, fallback, dest_field=None):
 """ Helper function used to set the fallback attributes of an element when they are defined by the configuration as "None" or "-". """


        if dest_field is None:
            dest_field = src_field
        if isinstance(fallback, six.string_types):
            fallback = elements[fallback]

        attrs = elements[src_field]
        elements[dest_field] = (
            attrs[0] if attrs[0] is not None else fallback[0],
            attrs[1] if attrs[1] is not None else fallback[1],
            attrs[2] if attrs[2] is not None else fallback[2])