Currently, when we are parsing a match expression, if we are in a position where there can be a $expr object, we will parse an object as a $expr if it contains exactly one field called $expr. Otherwise, it is treated as a literal object. However, this could cause an issue in the future if we add an additional option to $expr, say $option. Then in 3.6, we would parse {a: {$gt: {$expr: 5, $option: true}}} as comparison to the literal object {$expr: 5, $option: true}, but in 3.x, this would be treated as $expr with an option. Instead, in 3.6, we should fail to parse a $expr that has more than one field.