Reduced memory allocations

src/build/expression/Expression.go

@@ -28,6 +28,10 @@ func NewLeaf(t token.Token) *Expression {
 
 // AddChild adds a child to the expression.
 func (expr *Expression) AddChild(child *Expression) {
+	if expr.Children == nil {
+		expr.Children = make([]*Expression, 0, 2)
+	}
+
 	expr.Children = append(expr.Children, child)
 	child.Parent = expr
 }