def SUB_STR(string, start, length):
return {'<STR_LIT>': [string, start, length]}<EOL>
Returns a substring of a string, starting at a specified index position
and including the specified number of characters.
https://docs.mongodb.com/manual/reference/operator/aggregation/substr/
for more details
:param string: The string or expression of string
:param start: 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