[racket] fingertree / nested datatype

I've been reading about fingertrees, and I figure the best way to
understand is to implement. This is my first experience with typed
racket. Are nested datatypes supported?

This is my first try:

typed-fingertree.rkt:19:48: Type Checker: Structure type constructor
Deep applied to non-regular arguments ((U (Node2 a) (Node3 a)))

#lang typed/racket

(struct: (a) Digit1 ((v0 : a)))
(struct: (a) Digit2 ((v0 : a) (v1 : a)))
(struct: (a) Digit3 ((v0 : a) (v1 : a) (v2 : a)))
(struct: (a) Digit4 ((v0 : a) (v1 : a) (v2 : a) (v3 : a)))
(define-type (Digit a) (U (Digit1 a) (Digit2 a) (Digit3 a) (Digit4 a)))

(struct: (a) Node2 ((v0 : a) (v1 : a)))
(struct: (a) Node3 ((v0 : a) (v1 : a) (v2 : a)))
(define-type (Node a) (U (Node2 a) (Node3 a)))

(struct: Empty ())
(struct: (a) Single ((v : a)))
(struct: (a) Deep ((left : (Digit a))
		   (spine : (Fingertree (Node a)))
		   (right : (Digit a))))

(define-type (Fingertree a) (U Empty (Single a) (Deep a)))

Thank you!

-- 
Anthony Carrico

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