x: undefined; cannot reference undefined identifier.

(define-syntax-rule (mylet x a body)
  ((lambda (x) body) a))

(define-syntax mylet
  (syntax-rules ()
    ((_ x a body)
     ((lambda (x) body) a))))

(define-syntax-rule (let ((id val) ...) body ...)
  ((lambda (id ...) body ...) val ...))