diff --git a/Cargo.lock b/Cargo.lock
index ffdf0a46..5dc26c9f 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -467,6 +467,7 @@ dependencies = [
  "itertools",
  "num-bigint 0.3.2",
  "num-traits",
+ "parking_lot",
  "rustpython-parser",
  "test-case",
 ]
diff --git a/nac3core/Cargo.toml b/nac3core/Cargo.toml
index 09ebf724..e7081bf5 100644
--- a/nac3core/Cargo.toml
+++ b/nac3core/Cargo.toml
@@ -11,6 +11,7 @@ inkwell = { git = "https://github.com/TheDan64/inkwell", branch = "master", feat
 rustpython-parser = { git = "https://github.com/RustPython/RustPython", branch = "master" }
 itertools = "0.10.1"
 crossbeam = "0.8.1"
+parking_lot = "0.11.1"
 
 [dev-dependencies]
 test-case = "1.2.0"
diff --git a/nac3core/src/typecheck/top_level.rs b/nac3core/src/typecheck/top_level.rs
index 5d63a920..776e7cb8 100644
--- a/nac3core/src/typecheck/top_level.rs
+++ b/nac3core/src/typecheck/top_level.rs
@@ -1,8 +1,8 @@
-use std::collections::HashMap;
+use std::{collections::HashMap, sync::Arc};
 
-use super::typedef::{SharedUnifier, Type};
+use super::typedef::{SharedUnifier, Type, Unifier};
 use crossbeam::queue::SegQueue;
-use crossbeam::sync::ShardedLock;
+use parking_lot::RwLock;
 use rustpython_parser::ast::Stmt;
 
 pub struct DefinitionId(usize);
@@ -45,7 +45,12 @@ pub struct CodeGenTask {
 }
 
 pub struct TopLevelContext {
-    pub definitions: Vec<ShardedLock<TopLevelDef>>,
+    pub definitions: Vec<RwLock<TopLevelDef>>,
     pub unifiers: Vec<SharedUnifier>,
     pub codegen_queue: SegQueue<CodeGenTask>,
 }
+
+pub struct WorkerContext {
+    pub unifier: Unifier,
+    pub top_level_ctx: Arc<RwLock<TopLevelContext>>,
+}