substrate nodes in subcircuits that don't make connections to the
substrate (such as arrays of pFETs). This is done under the
restriction that ext2spice is being called without generating
parasitics (otherwise the connection to substrate is valid), as it
would be when extracting for LVS. The ground node must not appear
as a connection to any device. This is then propagated up the
hierarchy such that if none of a subcell's descendents connect to
the substrate, then neither does the subcell.
15f081b4da