Production rules assume that a pull-up rule is implicitly connected to some high power supply (often Vdd) and pull-down rules are rooted at some low supply (often GND). The simulated interpretation of rules is agnostic to the actual supply used. When left unspeciifed, rules by default connect to the implicit supplies for each process, !Vdd and !GND. See Implicit Ports. For netlist back-ends, however, the way to override the supply nodes for a block of production rules is to pass in replacement supplies: