import hashlib


def node_merkle_tree_hash(l, r):
    """
    Calculate the Merkle Tree Node Hash for an existing left and right hash
    (HASH(chr(1) || l || r)).
    l the left node hash.
    r the right node hash.
    Return the node hash for the combination.
    """
    return hashlib.sha256(chr(1) + l + r).digest()