[dragonwriter]

> how is a user to find out whether square really accepts a matrix?

How do you find out that a function exists, what it accepts, and what it does normally?

> From comments? They are again limited by the authors imagination.

Sure, but an author which much experience with duck typing will typically write documentation that specified that expected methods (informal structural “types”) rather than nominal types.

With the static equivalent (protocols), it’s conceivable that this is also discoverable via tooling.