Ephemeral nodes
In PQL, ephemeral nodes exist only temporarily, or for a very short time.
They can be created accidently by running a query like the one below, in
which the node 4 has no existence outside of the query itself.
The pound # sign in /#id
avoids accidently creating ephemeral nodes with the id function.
In the example below, the ephemeral nodes of 4 exist only because
the query points to them. They don't exist outside of the query.
