diff --git a/Cargo.lock b/Cargo.lock
index 6c55c9ba..ef3f28a0 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -23,9 +23,9 @@ checksum = "cdb031dd78e28731d87d56cc8ffef4a8f36ca26c38fe2de700543e627f8a464a"
 
 [[package]]
 name = "az"
-version = "1.0.0"
+version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e9bcd47d94aa4eb8c076b50fc61a75020789394ffb9bd74a180b3379130f6569"
+checksum = "d84e1d907bfc5795a6addb95ef8666141ee73c8f2f5250ff2a46bf4e4f4aec8a"
 
 [[package]]
 name = "base64"
@@ -470,9 +470,9 @@ dependencies = [
 
 [[package]]
 name = "libc"
-version = "0.2.84"
+version = "0.2.85"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1cca32fa0182e8c0989459524dc356b8f2b5c10f1b9eb521b7d182c03cf8c5ff"
+checksum = "7ccac4b00700875e6a07c6cde370d44d32fa01c5a65cdd2fca6858c479d28bb3"
 
 [[package]]
 name = "libsodium-sys"
@@ -1103,9 +1103,9 @@ dependencies = [
 
 [[package]]
 name = "ring"
-version = "0.16.19"
+version = "0.16.20"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "024a1e66fea74c66c66624ee5622a7ff0e4b73a13b4f5c326ddb50c708944226"
+checksum = "3053cf52e236a3ed746dfc745aa9cacf1b791d846bdaf412f60a8d7d6e17c8fc"
 dependencies = [
  "cc",
  "libc",
diff --git a/Cargo.toml b/Cargo.toml
index 084990ef..c49eebcf 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -11,6 +11,9 @@ keywords = ["prolog", "prolog-interpreter", "prolog-system"]
 categories = ["command-line-utilities"]
 build = "build.rs"
 
+[workspace]
+members = ["crates/prolog_parser"]
+
 [build-dependencies]
 indexmap = "1.0.2"
 
diff --git a/src/machine/machine_state_impl.rs b/src/machine/machine_state_impl.rs
index 277caa08..1b62431e 100644
--- a/src/machine/machine_state_impl.rs
+++ b/src/machine/machine_state_impl.rs
@@ -162,6 +162,14 @@ impl MachineState {
     }
 
     fn bind_with_occurs_check(&mut self, r: Ref, addr: Addr) {
+        if let Ref::StackCell(..) = r {
+            // local variable optimization -- r cannot occur in the
+            // data structure bound to addr, so don't bother
+            // traversing it.
+            self.bind(r, addr);
+            return;
+        }
+
         let mut fail = false;
 
         for addr in self.acyclic_pre_order_iter(addr) {