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#lang racket/base
(require racket/set)
(provide len)
(define (len x)
(cond
[(list? x) (length x)]
[(string? x) (string-length x)]
[(symbol? x) (len (symbol->string x))]
[(vector? x) (vector-length x)]
[(hash? x) (len (hash-keys x))]
[(integer? x) (len (number->string x))]
[(set? x) (len (set->list x))]
[else (error "len: can’t calculate length of" x)]))