diff --git a/tests/surface-stx.rkt b/tests/surface-stx.rkt
new file mode 100644
index 0000000..9a7a905
--- /dev/null
+++ b/tests/surface-stx.rkt
@@ -0,0 +1,126 @@
+#lang racket/base
+
+(require "../testing.rkt")
+
+
+(define (check-exn-syntax-datum-equal datum thunk)
+  (check-exn
+   (lambda (exn) (equal? (syntax->datum (car (exn:fail:syntax-exprs exn)))
+                         datum))
+   thunk))
+
+(define-syntax-rule
+  (check-syntax-error-datum datum expr)
+  (check-exn-syntax-datum-equal
+   datum
+   (lambda ()
+     (convert-compile-time-error
+      expr))))
+
+(begin-for-syntax
+
+  (define (source-location-contained? inner outer)
+    (and (equal? (syntax-source inner)
+                 (syntax-source outer))
+         (>= (syntax-position inner)
+             (syntax-position outer))
+         (<= (+ (syntax-position inner)
+                (syntax-span inner))
+             (+ (syntax-position outer)
+                (syntax-span outer)))))
+
+  ;; Example: (and g) → g
+  ;; This would naively highlight (and g), but in this case
+  ;; we want to highlight g instead. So, we check whether
+  ;; one expression is contained in the other, and if so,
+  ;; keep the srcloc of the inner one, to handle this.
+  (define (propagate-syntax-loc f)
+    (λ (stx)
+      (let ([res (f stx)])
+        (datum->syntax res  ; lexical context
+                       ;; datum
+                       (syntax-e res)
+                       ;; for srcloc
+                       (if (source-location-contained? res stx)
+                           res
+                           stx)
+                       ;; for properties
+                       res))))
+
+  (define (raise-surface-syntax-error message form)
+    (raise-syntax-error #f message (syntax-surface-stx form))))
+
+
+(syntax-spec
+  (extension-class flow-macro)
+  
+  (nonterminal floe
+    #:allow-extension flow-macro
+    id
+    (err f:floe)
+    (thread f:floe ...))
+
+  (host-interface/expression
+   (flow f:floe)
+   #'(compile-flow f)))
+
+(begin-for-syntax
+  (define (built-in-flow-macro f)
+    (flow-macro
+     (propagate-syntax-loc
+      f))))
+
+(define-syntax compile-flow
+  (syntax-parser
+    #:datum-literals (id err thread)
+    [(_ (thread f ...))
+     #'(list (compile-flow f) ...)]
+    [(_ id)
+     #'(lambda (x) x)]
+    [(_ (~and form (err f)))
+     (raise-surface-syntax-error "error" #'form)]))
+
+(flow (thread (err (thread id))))
+
+(flow (thread (err-id)))
+->
+(flow (thread (err id)))
+
+#;(check-syntax-error-datum
+ '(err (thread id))
+ (flow (thread (err (thread id)))))
+
+(define-syntax err-id
+  (flow-macro
+   (syntax-rules ()
+     [(_)
+      (err id)])))
+
+(define-syntax err-id/built-in
+  (built-in-flow-macro
+   (syntax-rules ()
+     [(_)
+      (err id)])))
+
+#;(flow (thread (err-id)))
+
+#;(convert-compile-time-error
+ (flow (thread (err-id/built-in))))
+
+#;(check-syntax-error-datum
+   '(err-id/built-in)
+   (flow (thread (err-id/built-in))))
+
+#|
+(define-syntax id-id
+  (flow-macro
+   (syntax-rules ()
+     [(_)
+      (thread id id)])))
+
+    
+;; Wrong!
+(check-syntax-error-datum
+ '(err (thread id id))
+ (flow (thread (err (id-id)))))
+|#