diff --git a/lib/ast.js b/lib/ast.js
index f3df78fe..6cc97447 100644
--- a/lib/ast.js
+++ b/lib/ast.js
@@ -591,10 +591,20 @@ var AST_New = DEFNODE("New", null, {
 var AST_Seq = DEFNODE("Seq", "car cdr", {
     $documentation: "A sequence expression (two comma-separated expressions)",
     $propdoc: {
-        car: "[AST_Node] first element in sequence",
-        cdr: "[AST_Node] second element in sequence"
+        car: "[AST_Node] first element in sequence (should not be instanceof AST_Seq or null)",
+        cdr: "[AST_Node] second element in sequence (should not be null)"
     },
     $cons: function(x, y) {
+        // TODO: Remove these sanity checks at the end of PR
+        if (!x) {
+            throw new Error('AST_Seq.car missing');
+        }
+        if (!y) {
+            throw new Error('AST_Seq.cdr missing');
+        }
+        if (x instanceof AST_Seq) {
+            throw new Error('AST_Seq.car cannot be AST_Seq');
+        }
         var seq = new AST_Seq(x);
         seq.car = x;
         seq.cdr = y;
@@ -603,53 +613,53 @@ var AST_Seq = DEFNODE("Seq", "car cdr", {
     $from_array: function(array) {
         if (array.length == 0) return null;
         if (array.length == 1) return array[0].clone();
-        var list = null;
-        for (var i = array.length; --i >= 0;) {
-            list = AST_Seq.cons(array[i], list);
-        }
-        var p = list;
-        while (p) {
-            if (p.cdr && !p.cdr.cdr) {
-                p.cdr = p.cdr.car;
-                break;
+        var i = array.length - 1;
+        var list = array[i];
+        while (--i >= 0) {
+            var node = array[i];
+            if (node instanceof AST_Seq) {
+                node.add(list);
+                list = node;
+            } else {
+                list = AST_Seq.cons(array[i], list);
             }
-            p = p.cdr;
         }
         return list;
     },
     to_array: function() {
-        var p = this, a = [];
-        while (p) {
+        var a = [];
+        var p = this;
+        do {
             a.push(p.car);
-            if (p.cdr && !(p.cdr instanceof AST_Seq)) {
-                a.push(p.cdr);
-                break;
-            }
             p = p.cdr;
-        }
+        } while (p instanceof AST_Seq);
+        a.push(p);
         return a;
     },
-    add: function(node) {
+    tail: function() {
         var p = this;
-        while (p) {
-            if (!(p.cdr instanceof AST_Seq)) {
-                var cell = AST_Seq.cons(p.cdr, node);
-                return p.cdr = cell;
-            }
+        while (p.cdr instanceof AST_Seq) {
             p = p.cdr;
         }
+        return p;
+    },
+    add: function(node) {
+        var p = this.tail();
+        p.cdr = AST_Seq.cons(p.cdr, node);
     },
     len: function() {
-        if (this.cdr instanceof AST_Seq) {
-            return this.cdr.len() + 1;
-        } else {
-            return 2;
+        var i = 2;
+        var p = this.cdr;
+        while (p instanceof AST_Seq) {
+            p = p.cdr;
+            i++;
         }
+        return i;
     },
     _walk: function(visitor) {
         return visitor._visit(this, function(){
             this.car._walk(visitor);
-            if (this.cdr) this.cdr._walk(visitor);
+            this.cdr._walk(visitor);
         });
     }
 });
diff --git a/lib/compress.js b/lib/compress.js
index 53833115..e53e6ba8 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -959,7 +959,7 @@ merge(Compressor.prototype, {
             return this.operator == "=" && this.right.is_boolean();
         });
         def(AST_Seq, function(){
-            return this.cdr.is_boolean();
+            return this.tail().cdr.is_boolean();
         });
         def(AST_True, return_true);
         def(AST_False, return_true);
@@ -982,7 +982,7 @@ merge(Compressor.prototype, {
             return (this.operator == "=" || this.operator == "+=") && this.right.is_string(compressor);
         });
         def(AST_Seq, function(compressor){
-            return this.cdr.is_string(compressor);
+            return this.tail().cdr.is_string(compressor);
         });
         def(AST_Conditional, function(compressor){
             return this.consequent.is_string(compressor) && this.alternative.is_string(compressor);
@@ -1459,8 +1459,12 @@ merge(Compressor.prototype, {
             return !compressor.option("pure_getters");
         });
         def(AST_Seq, function(compressor){
-            return this.car.has_side_effects(compressor)
-                || this.cdr.has_side_effects(compressor);
+            var p = this;
+            do {
+                if (p.car.has_side_effects(compressor)) return true;
+                p = p.cdr;
+            } while (p instanceof AST_Seq);
+            return p.has_side_effects(compressor);
         });
     })(function(node, func){
         node.DEFMETHOD("has_side_effects", func);
@@ -2669,12 +2673,11 @@ merge(Compressor.prototype, {
 
     AST_Unary.DEFMETHOD("lift_sequences", function(compressor){
         if (compressor.option("sequences")) {
-            if (this.expression instanceof AST_Seq) {
-                var seq = this.expression;
-                var x = seq.to_array();
-                this.expression = x.pop();
-                x.push(this);
-                seq = AST_Seq.from_array(x).transform(compressor);
+            var seq = this.expression;
+            if (seq instanceof AST_Seq) {
+                var p = seq.tail();
+                this.expression = p.cdr;
+                p.cdr = this.transform(compressor);
                 return seq;
             }
         }
@@ -2687,8 +2690,9 @@ merge(Compressor.prototype, {
 
     OPT(AST_UnaryPrefix, function(self, compressor){
         self = self.lift_sequences(compressor);
-        var e = self.expression;
+        if (!(self instanceof AST_UnaryPrefix)) return self;
         if (compressor.option("booleans") && compressor.in_boolean_context()) {
+            var e = self.expression;
             switch (self.operator) {
               case "!":
                 if (e instanceof AST_UnaryPrefix && e.operator == "!") {
@@ -2726,22 +2730,20 @@ merge(Compressor.prototype, {
 
     AST_Binary.DEFMETHOD("lift_sequences", function(compressor){
         if (compressor.option("sequences")) {
-            if (this.left instanceof AST_Seq) {
-                var seq = this.left;
-                var x = seq.to_array();
-                this.left = x.pop();
-                x.push(this);
-                seq = AST_Seq.from_array(x).transform(compressor);
+            var seq = this.left;
+            if (seq instanceof AST_Seq) {
+                var p = seq.tail();
+                this.left = p.cdr;
+                p.cdr = this.transform(compressor);
                 return seq;
             }
-            if (this.right instanceof AST_Seq
+            seq = this.right;
+            if (seq instanceof AST_Seq
                 && this instanceof AST_Assign
                 && !has_side_effects_or_prop_access(this.left, compressor)) {
-                var seq = this.right;
-                var x = seq.to_array();
-                this.right = x.pop();
-                x.push(this);
-                seq = AST_Seq.from_array(x).transform(compressor);
+                var p = seq.tail();
+                this.right = p.cdr;
+                p.cdr = this.transform(compressor);
                 return seq;
             }
         }
@@ -2809,6 +2811,7 @@ merge(Compressor.prototype, {
             }
         }
         self = self.lift_sequences(compressor);
+        if (!(self instanceof AST_Binary)) return self;
         if (compressor.option("comparisons")) switch (self.operator) {
           case "===":
           case "!==":
@@ -3078,6 +3081,7 @@ merge(Compressor.prototype, {
     var ASSIGN_OPS_COMMUTATIVE = [ '*', '|', '^', '&' ];
     OPT(AST_Assign, function(self, compressor){
         self = self.lift_sequences(compressor);
+        if (!(self instanceof AST_Assign)) return self;
         if (self.operator == "=" && self.left instanceof AST_SymbolRef && self.right instanceof AST_Binary) {
             // x = expr1 OP expr2
             if (self.right.left instanceof AST_SymbolRef
@@ -3100,12 +3104,16 @@ merge(Compressor.prototype, {
     });
 
     OPT(AST_Conditional, function(self, compressor){
-        if (!compressor.option("conditionals")) return self;
-        if (self.condition instanceof AST_Seq) {
-            var car = self.condition.car;
-            self.condition = self.condition.cdr;
-            return AST_Seq.cons(car, self);
+        if (compressor.option("sequences")) {
+            var seq = self.condition;
+            if (seq instanceof AST_Seq) {
+                var p = seq.tail();
+                self.condition = p.cdr;
+                p.cdr = self.transform(compressor);
+                return seq;
+            }
         }
+        if (!compressor.option("conditionals")) return self;
         var cond = self.condition.evaluate(compressor);
         if (cond.length > 1) {
             if (cond[1]) {
@@ -3188,7 +3196,7 @@ merge(Compressor.prototype, {
             && consequent.equivalent_to(alternative)) {
             var consequent_value = consequent.evaluate(compressor)[0];
             if (self.condition.has_side_effects(compressor)) {
-                return AST_Seq.from_array([self.condition, consequent_value]);
+                return AST_Seq.cons(self.condition, consequent_value);
             } else {
                 return consequent_value;
             }
diff --git a/lib/output.js b/lib/output.js
index 4a0a1e0e..577b60f8 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -1106,14 +1106,12 @@ function OutputStream(options) {
 
     AST_Seq.DEFMETHOD("_do_print", function(output){
         this.car.print(output);
-        if (this.cdr) {
-            output.comma();
-            if (output.should_break()) {
-                output.newline();
-                output.indent();
-            }
-            this.cdr.print(output);
+        output.comma();
+        if (output.should_break()) {
+            output.newline();
+            output.indent();
         }
+        this.cdr.print(output);
     });
     DEFPRINT(AST_Seq, function(self, output){
         self._do_print(output);
diff --git a/test/compress/typeof.js b/test/compress/typeof.js
index fb391573..cc281011 100644
--- a/test/compress/typeof.js
+++ b/test/compress/typeof.js
@@ -29,6 +29,7 @@ typeof_in_boolean_context: {
         booleans     : true,
         evaluate     : true,
         conditionals : true,
+        sequences    : true,
     };
     input: {
         function f1(x) { return typeof x ? "yes" : "no"; }
@@ -40,8 +41,7 @@ typeof_in_boolean_context: {
     expect: {
         function f1(x) { return "yes"; }
         function f2() { return g(), "Yes"; }
-        foo();
-        !(console.log(1), !0);
-        var a = !(console.log(2), !0);
+        foo(), console.log(1), !1;
+        var a = (console.log(2), !1);
     }
 }