Optional aspect can be applied to a member of an interface on the Cocoa platform to make it optional, meaning that classes implementing the interface will not need to provide an implementation. Callers working with an interface reference will need to check using the Cocoa
respondsToSelector API whether an optional member is supported.
The aspect is mostly used for C# and Java. In both Oxygene and Swift, the
optional keyword can be used to express the same notion.
Defined by the core compiler.
optionalkeyword in Oxygene Interfaces