Your code (`when` that binds `it`) translates to: (define-syntax (awhen stx) (syntax-case stx () ((_ expr body ...) (with-syntax ((it (datum

Your code (`when` that binds `it`) translates to: (define-syntax (awhen stx) (syntax-case stx () ((_ expr body ...) (with-syntax ((it (datum->syntax stx 'it))) #`(let ((it expr)) (when it body ...)))))) I find it a bit hairy, but mostly sane. It's all about being hygienic. Notice how you explicitly place the `it` identifier into the caller's context but keep the usual let and when?