The term "parent" as it has previously been used in this module is
confusing, because it referred not to the actual parent of a leaf note,
but to the *sibling* of the leaf's future parent. "Ommer" is a
gender-neutral term for the sibling of a parent, so it is ideal
for use in this context.
For some applications, it is not necessary to include the
full range of witnessing and checkpointing operations. The
Frontier trait captures the minimal operations required:
appending leaves to the tree and constructing the root
of the tree by hashing against empty nodes.