def FIRST(expression):
return {'<STR_LIT>': expression}<EOL>
Returns the value that results from applying an expression to the first document in a group of documents that share the same group by key.
Only meaningful when documents are in a defined order.
See https://docs.mongodb.com/manual/reference/operator/aggregation/first/
for more details
:param expression: expression or variables
:return: Aggregation operator