7.9. rank-parent: What taxon is the parent
of a new taxon of a given rank?
When a new taxon is being added to the tree, this function
describes the location of its parent node. When the tree is
empty, as a special case the parent is signified by None. Otherwise, it is the last node in the path
from the root node, through each last child, until we reach a
a node that has a shallower depth than the new child but
has either no children or children at that depth or below.
nomcompile3
# rank-parent(taxaTree, rankCode) ==
# if (taxaTree has no taxa) or
# (taxaTree has no nodes with depth < rankCode's depth ->
# None
# else ->
# the last (relative to preorder) node in taxaTree with
# depth < rankCode's depth ]
