func ServeError(rw http.ResponseWriter, r *http.Request, err error) {
    rw.Header().Set("Content-Type", "application/json")
    switch e := err.(type) {
    case *CompositeError:
        er := flattenComposite(e)
        // strips composite errors to first element only
        if len(er.Errors) > 0 {
            ServeError(rw, r, er.Errors[0])
        } else {
            // guard against empty CompositeError (invalid construct)
            ServeError(rw, r, nil)
        }
    case *MethodNotAllowedError:
        rw.Header().Add("Allow", strings.Join(err.(*MethodNotAllowedError).Allowed, ","))
        rw.WriteHeader(asHTTPCode(int(e.Code())))
        if r == nil || r.Method != http.MethodHead {
            _, _ = rw.Write(errorAsJSON(e))
        }
    case Error:
        value := reflect.ValueOf(e)
        if value.Kind() == reflect.Ptr && value.IsNil() {
            rw.WriteHeader(http.StatusInternalServerError)
            _, _ = rw.Write(errorAsJSON(New(http.StatusInternalServerError, "Unknown error")))
            return
        }
        rw.WriteHeader(asHTTPCode(int(e.Code())))
        if r == nil || r.Method != http.MethodHead {
            _, _ = rw.Write(errorAsJSON(e))
        }
    case nil:
        rw.WriteHeader(http.StatusInternalServerError)
        _, _ = rw.Write(errorAsJSON(New(http.StatusInternalServerError, "Unknown error")))
    default:
        rw.WriteHeader(http.StatusInternalServerError)
        if r == nil || r.Method != http.MethodHead {
            _, _ = rw.Write(errorAsJSON(New(http.StatusInternalServerError, err.Error())))
        }
    }