diff --git a/src/fh_builtins_arith.c b/src/fh_builtins_arith.c
index c068ef5..8660780 100644
--- a/src/fh_builtins_arith.c
+++ b/src/fh_builtins_arith.c
@@ -205,7 +205,11 @@ static enum fh_error w_within(struct fh_thread_s *fh, const struct fh_word_s *w)
   // (t-l) (h-l) | U<
   // =within
   
-  bool within = 0; // TODO
+  const int32_t ti = (int32_t)test;
+  const int32_t li = (int32_t)low;
+  const int32_t hi = (int32_t)high;
+  
+  const bool within = (uint32_t)(ti-li) < (uint32_t)(hi-li);
   
   TRY(ds_push(fh, TOBOOL(within)));
   return FH_OK;
@@ -517,7 +521,7 @@ static enum fh_error w_um_mod(struct fh_thread_s *fh, const struct fh_word_s *w)
   TRY(ds_pop(fh, &div));
   TRY(ds_pop_dw(fh, &num));
 
-  uint64_t res = num / (uint64_t)div; // TODO verify this may be wrong
+  uint64_t res = num / (uint64_t)div;
   uint64_t rem = num % (uint64_t)div;
 
   if ((uint64_t)(uint32_t)rem != rem) {