FreeList displacement???

[danluu]

    // TODO: Perhaps this should return a Data*.
    // If nothing is displaced, returns nullptr.
    // Otherwise returns Data* for displaced.
    template <unsigned SIZE>
    void ExpressionTree::FreeList<SIZE>::Allocate(unsigned id)
    {

We believe this comment is irrelevant because this function doesn't displace anything. It's possible comment became obsolete when the concept of storage was added.