diff --git a/mk2rbc/expr.go b/mk2rbc/expr.go
index 81b31c7..ca48bd9 100644
--- a/mk2rbc/expr.go
+++ b/mk2rbc/expr.go
@@ -31,6 +31,12 @@
 	// Emit the code to copy the expression, otherwise we will end up
 	// with source and target pointing to the same list.
 	emitListVarCopy(gctx *generationContext)
+	// Return the expression, calling the transformer func for
+	// every expression in the tree. If the transformer func returns non-nil,
+	// its result is used in place of the expression it was called with in the
+	// resulting expression. The resulting starlarkExpr will contain as many
+	// of the same objects from the original expression as possible.
+	transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr
 }
 
 func maybeString(expr starlarkExpr) (string, bool) {
@@ -62,6 +68,14 @@
 	s.emit(gctx)
 }
 
+func (s *stringLiteralExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	if replacement := transformer(s); replacement != nil {
+		return replacement
+	} else {
+		return s
+	}
+}
+
 // Integer literal
 type intLiteralExpr struct {
 	literal int
@@ -85,6 +99,14 @@
 	s.emit(gctx)
 }
 
+func (s *intLiteralExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	if replacement := transformer(s); replacement != nil {
+		return replacement
+	} else {
+		return s
+	}
+}
+
 // Boolean literal
 type boolLiteralExpr struct {
 	literal bool
@@ -110,6 +132,14 @@
 	b.emit(gctx)
 }
 
+func (b *boolLiteralExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	if replacement := transformer(b); replacement != nil {
+		return replacement
+	} else {
+		return b
+	}
+}
+
 // interpolateExpr represents Starlark's interpolation operator <string> % list
 // we break <string> into a list of chunks, i.e., "first%second%third" % (X, Y)
 // will have chunks = ["first", "second", "third"] and args = [X, Y]
@@ -190,6 +220,19 @@
 	xi.emit(gctx)
 }
 
+func (xi *interpolateExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	argsCopy := make([]starlarkExpr, len(xi.args))
+	for i, arg := range xi.args {
+		argsCopy[i] = arg.transform(transformer)
+	}
+	xi.args = argsCopy
+	if replacement := transformer(xi); replacement != nil {
+		return replacement
+	} else {
+		return xi
+	}
+}
+
 type variableRefExpr struct {
 	ref       variable
 	isDefined bool
@@ -220,6 +263,14 @@
 	}
 }
 
+func (v *variableRefExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	if replacement := transformer(v); replacement != nil {
+		return replacement
+	} else {
+		return v
+	}
+}
+
 type toStringExpr struct {
 	expr starlarkExpr
 }
@@ -265,6 +316,15 @@
 	s.emit(gctx)
 }
 
+func (s *toStringExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	s.expr = s.expr.transform(transformer)
+	if replacement := transformer(s); replacement != nil {
+		return replacement
+	} else {
+		return s
+	}
+}
+
 type notExpr struct {
 	expr starlarkExpr
 }
@@ -291,6 +351,15 @@
 	n.emit(gctx)
 }
 
+func (n *notExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	n.expr = n.expr.transform(transformer)
+	if replacement := transformer(n); replacement != nil {
+		return replacement
+	} else {
+		return n
+	}
+}
+
 type eqExpr struct {
 	left, right starlarkExpr
 	isEq        bool // if false, it's !=
@@ -360,6 +429,16 @@
 	eq.emit(gctx)
 }
 
+func (eq *eqExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	eq.left = eq.left.transform(transformer)
+	eq.right = eq.right.transform(transformer)
+	if replacement := transformer(eq); replacement != nil {
+		return replacement
+	} else {
+		return eq
+	}
+}
+
 // variableDefinedExpr corresponds to Make's ifdef VAR
 type variableDefinedExpr struct {
 	v variable
@@ -388,6 +467,11 @@
 	v.emit(gctx)
 }
 
+func (v *variableDefinedExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	// TODO: VariableDefinedExpr isn't really an expression?
+	return v
+}
+
 type listExpr struct {
 	items []starlarkExpr
 }
@@ -442,6 +526,19 @@
 	l.emit(gctx)
 }
 
+func (l *listExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	itemsCopy := make([]starlarkExpr, len(l.items))
+	for i, item := range l.items {
+		itemsCopy[i] = item.transform(transformer)
+	}
+	l.items = itemsCopy
+	if replacement := transformer(l); replacement != nil {
+		return replacement
+	} else {
+		return l
+	}
+}
+
 func newStringListExpr(items []string) *listExpr {
 	v := listExpr{}
 	for _, item := range items {
@@ -505,6 +602,19 @@
 	c.emit(gctx)
 }
 
+func (c *concatExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	itemsCopy := make([]starlarkExpr, len(c.items))
+	for i, item := range c.items {
+		itemsCopy[i] = item.transform(transformer)
+	}
+	c.items = itemsCopy
+	if replacement := transformer(c); replacement != nil {
+		return replacement
+	} else {
+		return c
+	}
+}
+
 // inExpr generates <expr> [not] in <list>
 type inExpr struct {
 	expr  starlarkExpr
@@ -543,23 +653,33 @@
 	i.emit(gctx)
 }
 
+func (i *inExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	i.expr = i.expr.transform(transformer)
+	i.list = i.list.transform(transformer)
+	if replacement := transformer(i); replacement != nil {
+		return replacement
+	} else {
+		return i
+	}
+}
+
 type indexExpr struct {
 	array starlarkExpr
 	index starlarkExpr
 }
 
-func (ix indexExpr) emit(gctx *generationContext) {
+func (ix *indexExpr) emit(gctx *generationContext) {
 	ix.array.emit(gctx)
 	gctx.write("[")
 	ix.index.emit(gctx)
 	gctx.write("]")
 }
 
-func (ix indexExpr) typ() starlarkType {
+func (ix *indexExpr) typ() starlarkType {
 	return starlarkTypeString
 }
 
-func (ix indexExpr) eval(valueMap map[string]starlarkExpr) (res starlarkExpr, same bool) {
+func (ix *indexExpr) eval(valueMap map[string]starlarkExpr) (res starlarkExpr, same bool) {
 	newArray, isSameArray := ix.array.eval(valueMap)
 	newIndex, isSameIndex := ix.index.eval(valueMap)
 	if same = isSameArray && isSameIndex; same {
@@ -570,10 +690,20 @@
 	return
 }
 
-func (ix indexExpr) emitListVarCopy(gctx *generationContext) {
+func (ix *indexExpr) emitListVarCopy(gctx *generationContext) {
 	ix.emit(gctx)
 }
 
+func (ix *indexExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	ix.array = ix.array.transform(transformer)
+	ix.index = ix.index.transform(transformer)
+	if replacement := transformer(ix); replacement != nil {
+		return replacement
+	} else {
+		return ix
+	}
+}
+
 type callExpr struct {
 	object     starlarkExpr // nil if static call
 	name       string
@@ -642,6 +772,21 @@
 	cx.emit(gctx)
 }
 
+func (cx *callExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	if cx.object != nil {
+		cx.object = cx.object.transform(transformer)
+	}
+	argsCopy := make([]starlarkExpr, len(cx.args))
+	for i, arg := range cx.args {
+		argsCopy[i] = arg.transform(transformer)
+	}
+	if replacement := transformer(cx); replacement != nil {
+		return replacement
+	} else {
+		return cx
+	}
+}
+
 type ifExpr struct {
 	condition starlarkExpr
 	ifTrue    starlarkExpr
@@ -691,6 +836,92 @@
 	i.emit(gctx)
 }
 
+func (i *ifExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	i.condition = i.condition.transform(transformer)
+	i.ifTrue = i.ifTrue.transform(transformer)
+	i.ifFalse = i.ifFalse.transform(transformer)
+	if replacement := transformer(i); replacement != nil {
+		return replacement
+	} else {
+		return i
+	}
+}
+
+type identifierExpr struct {
+	name string
+}
+
+func (i *identifierExpr) eval(valueMap map[string]starlarkExpr) (res starlarkExpr, same bool) {
+	return i, true
+}
+
+func (i *identifierExpr) emit(gctx *generationContext) {
+	gctx.write(i.name)
+}
+
+func (i *identifierExpr) typ() starlarkType {
+	return starlarkTypeUnknown
+}
+
+func (i *identifierExpr) emitListVarCopy(gctx *generationContext) {
+	i.emit(gctx)
+}
+
+func (i *identifierExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	if replacement := transformer(i); replacement != nil {
+		return replacement
+	} else {
+		return i
+	}
+}
+
+type foreachExpr struct {
+	varName string
+	list    starlarkExpr
+	action  starlarkExpr
+}
+
+func (f *foreachExpr) eval(valueMap map[string]starlarkExpr) (res starlarkExpr, same bool) {
+	list, listSame := f.list.eval(valueMap)
+	action, actionSame := f.action.eval(valueMap)
+	same = listSame && actionSame
+	if same {
+		return f, same
+	} else {
+		return &foreachExpr{
+			varName: f.varName,
+			list:    list,
+			action:  action,
+		}, same
+	}
+}
+
+func (f *foreachExpr) emit(gctx *generationContext) {
+	gctx.write("[")
+	f.action.emit(gctx)
+	gctx.write(" for " + f.varName + " in ")
+	f.list.emit(gctx)
+	gctx.write("]")
+}
+
+func (f *foreachExpr) typ() starlarkType {
+	return starlarkTypeList
+}
+
+func (f *foreachExpr) emitListVarCopy(gctx *generationContext) {
+	f.emit(gctx)
+}
+
+func (f *foreachExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	f.list = f.list.transform(transformer)
+	f.action = f.action.transform(transformer)
+	if replacement := transformer(f); replacement != nil {
+		return replacement
+	} else {
+		return f
+	}
+}
+
 type badExpr struct {
 	errorLocation ErrorLocation
 	message       string
@@ -714,6 +945,14 @@
 	panic("implement me")
 }
 
+func (b *badExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
+	if replacement := transformer(b); replacement != nil {
+		return replacement
+	} else {
+		return b
+	}
+}
+
 func maybeConvertToStringList(expr starlarkExpr) starlarkExpr {
 	if xString, ok := expr.(*stringLiteralExpr); ok {
 		return newStringListExpr(strings.Fields(xString.literal))