<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def document_type2marc(self, key, value):
 """Populate the ``980`` MARC field."""

    if value in DOCUMENT_TYPE_REVERSE_MAP and DOCUMENT_TYPE_REVERSE_MAP[value]:
        return {'a': DOCUMENT_TYPE_REVERSE_MAP[value]}