def process_deffield_field(api_doc, docindex, tag, arg, descr):
    """Define a new custom field."""
    _check(api_doc, tag, arg, expect_arg=True)
    if api_doc.extra_docstring_fields is UNKNOWN:
        api_doc.extra_docstring_fields = []
    try:
        docstring_field = _descr_to_docstring_field(arg, descr)
        docstring_field.varnames.append("__%s__" % arg)
        api_doc.extra_docstring_fields.append(docstring_field)
    except __HOLE__, e:
        raise ValueError('Bad %s: %s' % (tag, e))
dataset/ETHPy150Open ardekantur/pyglet/tools/epydoc/epydoc/docstringparser.py/process_deffield_field