subscript(_:)

Appends a value to the list if non-nil, does nothing otherwise.

BSON.ListEncoder.swift:56
subscript<Encodable>(_: (Index) -> Void) -> Encodable? where Encodable : BSONEncodable { get set }

Why a subscript and not an append method? Because we often want to optionally append a value while building a list, and the subscript syntax is more convenient for that.