He writes more than this then deletes some of it. This is the minimum needed to run his "proof it works" example.
(define eval-expr
(lambda (expr env)
(pmatch expr
[,n (guard (number? n))
n]
[,x (guard (symbol? x))
(env x)]
[(lambda (,x) ,body)
(lambda (arg)
(eval-expr body (lambda (y)
(if (eq? x y)
arg
(env y)))))]
[(,rator ,rand)
((eval-expr rator env)
(eval-expr rand env))])))pmatch.scm is loaded but not made available. We can make an educated guess that this[0] is pmatch.scm.
His "proof it works" example using chez.
> (load "pmatch.scm")
> (eval-expr '((lambda (x) x) 5) (lambda (y) (error 'lookup "unbound")))
5