def STR_CASE_CMP(x, y):
return {'<STR_LIT>': [x, y]}<EOL>
Performs case-insensitive comparison of two strings. Returns
    1 if first string is “greater than” the second string.
    0 if the two strings are equal.
    -1 if the first string is “less than” the second string.

https://docs.mongodb.com/manual/reference/operator/aggregation/strcasecmp/
for more details
:param x: The first string or expression of string
:param y: The second string or expression of string
:return: Aggregation operator