Currently, the exact top N operators have the following syntax:

{$minN: {n: <expression for n>, output: <output expression>}}

While this might make sense in the case of accumulators, this syntax is potentially confusing when the operators are used as expressions because the 'output' field is really the input to the expression. As such, we should consider modifying the syntax to disambiguate what is meant by the 'output' field.