Some small things, that I recognized while browsing the documentation:

Title is wrong: No Trailing commas are present. ( Title is not shown in github view.)

Should be construction:

Print should be: // "value is [Jim] [(unknown)]"
Same for next line.

The override method is missing its implementation.

An inspect expr -> Type expression allows pattern matching using is.
I think for the reader it is still unclear what is expected on the right side of =. Particularly, I think the definition is not complete. Should it not be: inspect expr -> Type { is condition = statement; ...} My question would be: Can statement also be a body with a return?