Relative transformation (simplified)
For each node N in the output tree to which my X is transformed,
create a node Y as a Z of N.
For each node in the output tree to which my parent is transformed,
create a copy of me as a child of that node.
Call the node to be transformed the current match.
Call the node relative to which the current match is to be transformed
the leader node.
Call the nodes to which the leader node is transformed the leader
X = relationship of leader node to current match. Often parent or
Y = type and properties of node to be created. Often copy of node
Z = relationship of node to be created in output tree to a leader
destination: child, attribute, preceding sibling, following sibling.