diff --git a/usr/austin/eval/abort.go b/usr/austin/eval/abort.go
index 521e51652b5..53747f3e1b4 100644
--- a/usr/austin/eval/abort.go
+++ b/usr/austin/eval/abort.go
@@ -10,33 +10,29 @@ import (
 	"runtime";
 )
 
-// TODO(austin) This is not thread-safe.  We could include the abort
-// channel in the Frame structure, but then the Value methods need to
-// take the Frame.  However, passing something to the Value methods
-// might be necessary to generate back traces.
-var abortChan = make(chan os.Error)
-
-// Abort aborts the current computation.  If this is called within the
-// extent of a Try call, this immediately returns to the Try with the
-// given error.  If not, then this panic's.
-func Abort(e os.Error) {
-	if abortChan == nil {
-		panic("Abort: " + e.String());
+// Abort aborts the thread's current computation,
+// causing the innermost Try to return err.
+func (t *Thread) Abort(err os.Error) {
+	if t.abort == nil {
+		panicln("abort:", err.String());
 	}
-	abortChan <- e;
+	t.abort <- err;
 	runtime.Goexit();
 }
 
-// Try executes a computation with the ability to Abort.
-func Try(f func()) os.Error {
-	abortChan = make(chan os.Error);
+// Try executes a computation; if the computation
+// Aborts, Try returns the error passed to abort.
+func (t *Thread) Try(f func(t *Thread)) os.Error {
+	oc := t.abort;
+	c := make(chan os.Error);
+	t.abort = c;
 	go func() {
-		f();
-		abortChan <- nil;
+		f(t);
+		c <- nil;
 	}();
-	res := <-abortChan;
-	abortChan = nil;
-	return res;
+	err := <-c;
+	t.abort = oc;
+	return err;
 }
 
 type DivByZeroError struct {}
diff --git a/usr/austin/eval/expr.go b/usr/austin/eval/expr.go
index 4415c84ed28..cacfc61e126 100644
--- a/usr/austin/eval/expr.go
+++ b/usr/austin/eval/expr.go
@@ -967,7 +967,7 @@ func (a *exprInfo) compileIndexExpr(l, r *expr) *expr {
 		expr.genValue(func(t *Thread) Value {
 			l, r := lf(t), rf(t);
 			if r < 0 || r >= bound {
-				Abort(IndexError{r, bound});
+				t.Abort(IndexError{r, bound});
 			}
 			return l.Elem(r);
 		});
@@ -978,10 +978,10 @@ func (a *exprInfo) compileIndexExpr(l, r *expr) *expr {
 		expr.genValue(func(t *Thread) Value {
 			l, r := lf(t), rf(t);
 			if l.Base == nil {
-				Abort(NilPointerError{});
+				t.Abort(NilPointerError{});
 			}
 			if r < 0 || r >= l.Len {
-				Abort(IndexError{r, l.Len});
+				t.Abort(IndexError{r, l.Len});
 			}
 			return l.Base.Elem(r);
 		});
@@ -994,7 +994,7 @@ func (a *exprInfo) compileIndexExpr(l, r *expr) *expr {
 		expr.eval = func(t *Thread) uint64 {
 			l, r := lf(t), rf(t);
 			if r < 0 || r >= int64(len(l)) {
-				Abort(IndexError{r, int64(len(l))});
+				t.Abort(IndexError{r, int64(len(l))});
 			}
 			return uint64(l[r]);
 		}
@@ -1006,11 +1006,11 @@ func (a *exprInfo) compileIndexExpr(l, r *expr) *expr {
 			m := lf(t);
 			k := rf(t);
 			if m == nil {
-				Abort(NilPointerError{});
+				t.Abort(NilPointerError{});
 			}
 			e := m.Elem(k);
 			if e == nil {
-				Abort(KeyError{k});
+				t.Abort(KeyError{k});
 			}
 			return e;
 		});
@@ -1228,13 +1228,13 @@ func (a *exprInfo) compileBuiltinCallExpr(b *block, ft *FuncType, as []*expr) *e
 				// XXX(Spec) What if len or cap is
 				// negative?  The runtime panics.
 				if l < 0 {
-					Abort(NegativeLengthError{l});
+					t.Abort(NegativeLengthError{l});
 				}
 				c := l;
 				if capf != nil {
 					c = capf(t);
 					if c < 0 {
-						Abort(NegativeCapacityError{c});
+						t.Abort(NegativeCapacityError{c});
 					}
 					// XXX(Spec) What happens if
 					// len > cap?  The runtime
@@ -1293,7 +1293,7 @@ func (a *exprInfo) compileStarExpr(v *expr) *expr {
 		expr.genValue(func(t *Thread) Value {
 			v := vf(t);
 			if v == nil {
-				Abort(NilPointerError{});
+				t.Abort(NilPointerError{});
 			}
 			return v;
 		});
@@ -1863,7 +1863,7 @@ func (expr *Expr) Eval(f *Frame) (Value, os.Error) {
 	}
 	v := expr.e.t.Zero();
 	eval := genAssign(expr.e.t, expr.e);
-	err := Try(func() {eval(v, t)});
+	err := t.Try(func(t *Thread){eval(v, t)});
 	return v, err;
 }
 
diff --git a/usr/austin/eval/expr1.go b/usr/austin/eval/expr1.go
index dce004f40fb..e07ce86f391 100644
--- a/usr/austin/eval/expr1.go
+++ b/usr/austin/eval/expr1.go
@@ -379,11 +379,11 @@ func (a *expr) genBinOpQuo(l, r *expr) {
 	case *uintType:
 		lf := l.asUint();
 		rf := r.asUint();
-		a.eval = func(t *Thread) uint64 { l, r := lf(t), rf(t); if r == 0 { Abort(DivByZeroError{}) } return l / r }
+		a.eval = func(t *Thread) uint64 { l, r := lf(t), rf(t); if r == 0 { t.Abort(DivByZeroError{}) } return l / r }
 	case *intType:
 		lf := l.asInt();
 		rf := r.asInt();
-		a.eval = func(t *Thread) int64 { l, r := lf(t), rf(t); if r == 0 { Abort(DivByZeroError{}) } return l / r }
+		a.eval = func(t *Thread) int64 { l, r := lf(t), rf(t); if r == 0 { t.Abort(DivByZeroError{}) } return l / r }
 	case *idealIntType:
 		l := l.asIdealInt()();
 		r := r.asIdealInt()();
@@ -392,7 +392,7 @@ func (a *expr) genBinOpQuo(l, r *expr) {
 	case *floatType:
 		lf := l.asFloat();
 		rf := r.asFloat();
-		a.eval = func(t *Thread) float64 { l, r := lf(t), rf(t); if r == 0 { Abort(DivByZeroError{}) } return l / r }
+		a.eval = func(t *Thread) float64 { l, r := lf(t), rf(t); if r == 0 { t.Abort(DivByZeroError{}) } return l / r }
 	case *idealFloatType:
 		l := l.asIdealFloat()();
 		r := r.asIdealFloat()();
@@ -408,11 +408,11 @@ func (a *expr) genBinOpRem(l, r *expr) {
 	case *uintType:
 		lf := l.asUint();
 		rf := r.asUint();
-		a.eval = func(t *Thread) uint64 { l, r := lf(t), rf(t); if r == 0 { Abort(DivByZeroError{}) } return l % r }
+		a.eval = func(t *Thread) uint64 { l, r := lf(t), rf(t); if r == 0 { t.Abort(DivByZeroError{}) } return l % r }
 	case *intType:
 		lf := l.asInt();
 		rf := r.asInt();
-		a.eval = func(t *Thread) int64 { l, r := lf(t), rf(t); if r == 0 { Abort(DivByZeroError{}) } return l % r }
+		a.eval = func(t *Thread) int64 { l, r := lf(t), rf(t); if r == 0 { t.Abort(DivByZeroError{}) } return l % r }
 	case *idealIntType:
 		l := l.asIdealInt()();
 		r := r.asIdealInt()();
diff --git a/usr/austin/eval/func.go b/usr/austin/eval/func.go
index d13fbbed405..3bf52871dc9 100644
--- a/usr/austin/eval/func.go
+++ b/usr/austin/eval/func.go
@@ -4,11 +4,14 @@
 
 package eval
 
+import "os"
+
 /*
  * Virtual machine
  */
 
 type Thread struct {
+	abort chan os.Error;
 	pc uint;
 	// The execution frame of this function.  This remains the
 	// same throughout a function invocation.
@@ -18,13 +21,15 @@ type Thread struct {
 type code []func(*Thread)
 
 func (i code) exec(t *Thread) {
-	v := Thread{0, t.f};	// TODO: reuse t
+	opc := t.pc;
+	t.pc = 0;
 	l := uint(len(i));
-	for v.pc < l {
-		pc := v.pc;
-		v.pc++;
-		i[pc](&v);
+	for t.pc < l {
+		pc := t.pc;
+		t.pc++;
+		i[pc](t);
 	}
+	t.pc = opc;
 }
 
 /*
diff --git a/usr/austin/eval/gen.go b/usr/austin/eval/gen.go
index a9d088db8c3..a09ecfa70ae 100644
--- a/usr/austin/eval/gen.go
+++ b/usr/austin/eval/gen.go
@@ -91,12 +91,12 @@ var binOps = []Op{
 	Op{ Name: "Sub", Expr: "l - r", ConstExpr: "l.Sub(r)", Types: numbers },
 	Op{ Name: "Mul", Expr: "l * r", ConstExpr: "l.Mul(r)", Types: numbers },
 	Op{ Name: "Quo",
-		Body: "if r == 0 { Abort(DivByZeroError{}) } return l / r",
+		Body: "if r == 0 { t.Abort(DivByZeroError{}) } return l / r",
 		ConstExpr: "l.Quo(r)",
 		Types: numbers,
 	},
 	Op{ Name: "Rem",
-		Body: "if r == 0 { Abort(DivByZeroError{}) } return l % r",
+		Body: "if r == 0 { t.Abort(DivByZeroError{}) } return l % r",
 		ConstExpr: "l.Rem(r)",
 		Types: integers,
 	},
diff --git a/usr/austin/eval/stmt.go b/usr/austin/eval/stmt.go
index 50a776e35e4..e0d6ebeb30e 100644
--- a/usr/austin/eval/stmt.go
+++ b/usr/austin/eval/stmt.go
@@ -1281,7 +1281,7 @@ type Stmt struct {
 func (s *Stmt) Exec(f *Frame) os.Error {
 	t := new(Thread);
 	t.f = f;
-	return Try(func() {s.code.exec(t)});
+	return t.Try(func(t *Thread){s.code.exec(t)});
 }
 
 func CompileStmts(scope *Scope, stmts []ast.Stmt) (*Stmt, os.Error) {
diff --git a/usr/austin/eval/stmt_test.go b/usr/austin/eval/stmt_test.go
index f701d27f7aa..9a62b2e0f34 100644
--- a/usr/austin/eval/stmt_test.go
+++ b/usr/austin/eval/stmt_test.go
@@ -302,7 +302,7 @@ var stmtTests = []test {
 	SErr("type T struct { x int }; type U struct { x int }; var y struct { T; U }; y.x = 42", "ambiguous.*\tT\\.x\n\tU\\.x"),
 	SErr("type T struct { *T }; var x T; x.foo", "no field"),
 
-	//Val1("fib := func(int) int{return 0;}; fib = func(v int) int { if v < 2 { return 1 } return fib(v-1)+fib(v-2) }; i = fib(20)", "i", 0),
+	Val1("fib := func(int) int{return 0;}; fib = func(v int) int { if v < 2 { return 1 } return fib(v-1)+fib(v-2) }; i = fib(20)", "i", 10946),
 
 	// Make slice
 	Val2("x := make([]int, 2); x[0] = 42; i, i2 = x[0], x[1]", "i", 42, "i2", 0),