Leaf/parent indicator bit in tree hash?

[Bren2010]

When computing the tree hash, should there be an indicator bit for whether the value being hashed represents a leaf or parent node?

[bifurcation]

This doesn't seem like a terrible idea. You could either add a field uint8 leaf_or_parent = 0/1; to LeafNodeHashInput and ParentNodeHashInput, or define an enum and do the same, or make a select. For example:

enum { reserved(0), leaf(1), parent(2), (255) } TreeNodeType;

struct {
    TreeNodeType node_type;
    select (TreeHashInput.node_type) {
        case leaf: /* ParentNodeHashInput, as a struct or inlined */
        case parent: /* ParentNodeHashInput, as a struct or inlined */
    }
} TreeHashInput;