def SUB_STR_CP(string, index, count):
return {'<STR_LIT>': [string, index, count]}<EOL>
Returns the substring of a string.
The substring starts with the character at the specified UTF-8 code point (CP) index (zero-based)
in the string for the number of code points specified.
https://docs.mongodb.com/manual/reference/operator/aggregation/substrCP/
for more details
:param string: The string or expression of string
:param index: Indicates the starting point of the substring.
:param length: Can be any valid expression as long as it resolves to a non-negative integer or number
:return: Aggregation operator