I'm trying to use occurrence typing for (Vectorof Symbol) or (Boxof Symbol).

Creating a custom predicate for (Listof Symbol) seems to work:
#lang typed/racket

(: los? (-> Any Boolean : (Listof Symbol)))
(define los?
  (lambda (seq) (and (list? seq) (andmap symbol? seq))))

However I haven't been able to create one for vectors or boxes. None of the 
things I've tried will typecheck. Any pointers?

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