0. #lang racket
1. (define (foo xs)
2.  
3.             (cond [(null? xs) '()]
4.  
5.                   [(number? xs) xs]
6.  
7.                   [(list? xs) (cdr xs)]
8.  
9.                   [(boolean? xs) #t]
10.  
11.                   [#t #f]))
12. foo('(1 2 3 4))

=>

#<procedure:foo>

application: not a procedure;

expected a procedure that can be applied to arguments

given: '(1 2 3 4)

arguments...: [none]