diff --git a/src/fh_builtins_arith.c b/src/fh_builtins_arith.c
index da04faf..c068ef5 100644
--- a/src/fh_builtins_arith.c
+++ b/src/fh_builtins_arith.c
@@ -185,6 +185,31 @@ static enum fh_error w_greater(struct fh_thread_s *fh, const struct fh_word_s *w
   return FH_OK;
 }
 
+static enum fh_error w_within(struct fh_thread_s *fh, const struct fh_word_s *w)
+{
+  (void) w;
+  enum fh_error rv;
+  uint32_t test, low, high;
+  TRY(ds_pop(fh, &high));
+  TRY(ds_pop(fh, &low));
+  TRY(ds_pop(fh, &test));
+  
+  // This should work, according to the spec:
+  //  : WITHIN ( test low high -- flag ) OVER - >R - R> U< ;
+
+  // t l h | OVER
+  // t l h l | -
+  // t l (h-l) | >r
+  // t l | R:(h-l) | -
+  // (t-l) | R:(h-l) | R>
+  // (t-l) (h-l) | U<
+  // =within
+  
+  bool within = 0; // TODO
+  
+  TRY(ds_push(fh, TOBOOL(within)));
+  return FH_OK;
+}
 
 static enum fh_error w_u_less(struct fh_thread_s *fh, const struct fh_word_s *w)
 {
@@ -582,5 +607,6 @@ const struct name_and_handler fh_builtins_arith[] = {
     {"fm/mod",  w_fm_mod,         0, 0},
     {"sm/rem",  w_sm_rem,         0, 0},
     {"um/mod",  w_um_mod,         0, 0},
+    {"within",  w_within,         0, 0},
     { /* end marker */ }
 };