diff --git a/run b/run
index 247302c..c9748ac 100755
--- a/run
+++ b/run
@@ -196,11 +196,37 @@ step40() {
   echo "zig1.wasm updated for $TO_"
 }
 
+step50() {
+  local FROM="$CALL_USAGES"
+  local FROM_="$CALL_USAGES_"
+  local TO="$BUILTIN_TYPE"
+  local TO_="$BUILTIN_TYPE_"
+
+  echo "Updating zig1.wasm from $FROM_ -> $TO_ in ../zig-$TO_"
+  rm -fr "../zig-$TO_"
+  git archive --prefix=zig-$TO_/ $TO | tar -C .. -x
+  {
+    pushd "../zig-$TO_"
+    cp ../zig-$FROM_/stage1/zig1.wasm stage1/zig1.wasm
+    fullbuild
+    build/stage3/bin/zig build update-zig1
+    popd
+  }
+  echo "zig1.wasm updated for $TO_"
+}
+
 if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
   set -xeuo pipefail
-  step00
-  step10
-  step20
-  step30
-  step40
+  step=${1:-step00}
+  step=${step#step}
+
+  # works:
+  if [[ "$step" -le 00 ]] ; then step00; fi
+  if [[ "$step" -le 10 ]] ; then step10; fi
+  if [[ "$step" -le 20 ]] ; then step20; fi
+  if [[ "$step" -le 30 ]] ; then step30; fi
+  if [[ "$step" -le 40 ]] ; then step40; fi
+
+  # does not work:
+  if [[ "$step" -le 50 ]] ; then step50; fi
 fi