diff --git a/lib/infer.cora b/lib/infer.cora
index 6403bdf..5e76238 100644
--- a/lib/infer.cora
+++ b/lib/infer.cora
@@ -262,7 +262,7 @@
 (declare '- `(int -> (int -> int)))
 (declare '* `(int -> (int -> int)))
 (declare '= (let a (tvar) b (tvar) `(,a -> (,b -> bool))))
-(declare 'set (let a (tvar) ['symbol '-> [a '-> a]]))
+;; (declare 'set (let a (tvar) ['symbol '-> [a '-> a]]))
 (declare 'null? [(tvar) '-> 'bool])
 (declare 'cons? [(tvar) '-> 'bool])
 (declare '*type-env* ['list ['tuple 'symbol (tvar)]])
diff --git a/lib/toc.cora b/lib/toc.cora
index b0920a2..268bd57 100644
--- a/lib/toc.cora
+++ b/lib/toc.cora
@@ -516,27 +516,27 @@
 
 (set 'cora/lib/infer.*typecheck* false)
 
-(defun .preprocess (sexp)
- (let sexp1 (if (and (pair? sexp) (= 'begin (car sexp)))
-			 (cdr sexp)
-			 (cons sexp ()))
-	(begin
-		(.handle-import-eagerly sexp1)
-		(.split-type-and-code sexp1 () ()
-			(lambda (type code)
-				(if cora/lib/infer.*typecheck*
-					(cons 'begin (append type code))
-					(cons 'begin code)))))))
+(defun .preprocess (file-path pkg-str)
+  (let sexp (read-file-as-sexp file-path pkg-str)
+	(let sexp1 (if (and (pair? sexp) (= 'begin (car sexp)))
+					 (cdr sexp)
+					 (cons sexp ()))
+			(begin
+				(.handle-import-eagerly sexp1)
+				(.split-type-and-code sexp1 () ()
+					(lambda (type code)
+						(if cora/lib/infer.*typecheck*
+							(cons 'begin (append type code))
+							(cons 'begin code))))))))
 
 (defun .compile-to-c (from to pkg-str)
-  (let sexp (read-file-as-sexp from pkg-str)
-			(let sexp1 (.preprocess sexp)
-					(let input (macroexpand sexp1)
-						 (let bc (.compile input)
-						  (let stream (io.open-output-file to)
-							   (begin
-							(.generate-c stream bc)
-							(io.close-output-file stream))))))))
+	(let sexp1 (.preprocess from pkg-str)
+	 (let input (macroexpand sexp1)
+	  (let bc (.compile input)
+	   (let stream (io.open-output-file to)
+		(begin
+		 (.generate-c stream bc)
+		 (io.close-output-file stream)))))))
 
 
 ;; ============== utilities of eval0 ==========
diff --git a/src/gc.c b/src/gc.c
index 835e91c..79892e6 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -190,8 +190,11 @@ blockReset(struct Block *block) {
 		}
 }
 
+__thread struct GC* threadLocalGC;
+
 void
-gcInit(struct GC *gc, uintptr_t *baseStackAddr) {
+gcInit(uintptr_t *baseStackAddr) {
+		struct GC *gc = malloc(sizeof(struct GC));
 		gc->heap = NULL;
 		gc->state = gcStateNone;
 		gc->baseStackAddr = baseStackAddr;
@@ -211,6 +214,13 @@ gcInit(struct GC *gc, uintptr_t *baseStackAddr) {
 		for (int i=0; i<sizeClassSZ; i++) {
 				gc->sizeClass[i] = NULL;
 		}
+
+		threadLocalGC = gc;
+}
+
+struct GC*
+getGC() {
+		return threadLocalGC;
 }
 
 static struct heapArena*
@@ -324,8 +334,6 @@ gcRegistForType(uint8_t idx, gcFunc fn) {
   return true;
 }
 
-struct GC gc;
-
 static void
 gcQueueInit(struct GC *gc) {
   struct Block *b = blockNew(gc);
@@ -550,11 +558,11 @@ gcInuseSizeInc(struct GC *gc, int size) {
 }
 
 void
-writeBarrier(uintptr_t *slot, uintptr_t val) {
+writeBarrier(struct GC *gc, uintptr_t *slot, uintptr_t val) {
   // Yuasa-style deletion barrier
-  if (gc.state == gcStateIncremental) {
+  if (gc->state == gcStateIncremental) {
     uintptr_t old = *slot;
-    gcMark(&gc, old);
+    gcMark(gc, old);
   }
   *slot = val;
 }
diff --git a/src/gc.h b/src/gc.h
index 223af0f..78b8df5 100644
--- a/src/gc.h
+++ b/src/gc.h
@@ -37,17 +37,18 @@ typedef struct {
 #define ptr(x) ((void*)((x)&~TAG_PTR))
 #define tag(x) ((x) & TAG_MASK)
 
-extern struct GC gc;
-void gcInit(struct GC* gc, uintptr_t* mark);
+struct GC;
+
+void gcInit(uintptr_t* mark);
+struct GC *getGC();
+
 void* gcAlloc(struct GC* gc, int size);
 
-void writeBarrier(uintptr_t *slot, uintptr_t val);
+void writeBarrier(struct GC *gc, uintptr_t *slot, uintptr_t val);
 void gcMark(struct GC *gc, uintptr_t head);
 void gcInuseSizeInc(struct GC *gc, int size);
 
 typedef void (*gcFunc)(struct GC *gc, void* from);
 bool gcRegistForType(uint8_t type, gcFunc fn);
 
-extern struct GC gc;
-
 #endif
diff --git a/src/runtime.c b/src/runtime.c
index bade6ed..64b2a6c 100644
--- a/src/runtime.c
+++ b/src/runtime.c
@@ -856,7 +856,7 @@ registerAPI(struct registerModule* m, str pkg) {
 
 void
 coraInit(uintptr_t *mark) {
-  gcInit(&gc, mark);
+  gcInit(mark);
   typesInit();
   symQuote = intern("quote");
   symBackQuote = intern("backquote");
diff --git a/src/types.c b/src/types.c
index f3355f2..45f23e2 100644
--- a/src/types.c
+++ b/src/types.c
@@ -30,7 +30,7 @@ const char* typeNameX[8] = {
 void*
 newObj(scmHeadType tp, int sz) {
   // scmHead* p = malloc(sz);
-  scmHead* p = gcAlloc(&gc, sz);
+  scmHead* p = gcAlloc(getGC(), sz);
   assert(((Obj)p & TAG_PTR) == 0);
   p->type = tp;
   /* printf("alloc object -- %p %s\n", p, typeNameX[tp]); */
@@ -338,7 +338,7 @@ vectorSet(Obj vec, int idx, Obj val) {
   struct scmVector* v = ptr(vec);
   assert(v->head.type == scmHeadVector);
   assert(idx >= 0 && idx < v->size);
-  writeBarrier(&v->data[idx], val);
+  writeBarrier(getGC(), &v->data[idx], val);
   return vec;
 }