def CMP(x=None, y=None):
if x is None and y is None:<EOL><INDENT>return {'<STR_LIT>': []}<EOL><DEDENT>return {'<STR_LIT>': [x, y]}<EOL>
Compares two values and returns:
    -1 if the first value is less than the second.
    1 if the first value is greater than the second.
    0 if the two values are equivalent.
See https://docs.mongodb.com/manual/reference/operator/aggregation/cmp/
for more details
:param x: first value or expression
:param y: second value or expression
:return: Aggregation operator