Change:
For example, consider the following tree, where the "_" character represents a blank node: <del> In this tree, we can see all three of the above rules in play: </del> <ins> In this tree, we can see all of the above rules in play: </ins> <del> The resolution of node 5 is the list [CD] </del> <ins> The resolution of node 5 is the list [CD, C] </ins> The resolution of node 2 is the empty list [] <del> The resolution of node 3 is the list [A, CD] </del> <ins> The resolution of node 3 is the list [A, CD, C] </ins> <del> Every node, regardless of whether a node is blank or populated, has a corresponding _hash_ that summarizes the contents of the subtree </del> <ins> Every node, regardless of whether the node is blank or populated, has a corresponding _hash_ that summarizes the contents of the subtree </ins> below that node.  The rules for computing these hashes are described in tree-hashes.

Discussion:
This PR attempts to implement the "non-destructive add" proposal discussed at IETF 105. Instead of blanking nodes on add, we keep track of "unmerged leaves" below each intermediate node. This involves a bit more complexity in the tree state that is passed around, but avoids unnecessary blank nodes.
As discussed in the interim: Remove processing should be updated such that for each node in the removed node's direct path: If the removed leaf is in for the node, then just remove it from that list Otherwise, blank the node
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