Advanced ERC Features

The ERC can only verify simple relationships between networks and components. I'd like to extend it with

 - conditional pin types (allowing resistors and inductors to pass through power)
 - voltage-above-reference constraints (e.g. VCC pin must be +3V with reference to GND pin)
 - pull-up/pull-down resistors (pins requiring termination if unused, configuration pins)
 - pins that must be tied together (i.e. technically only one pin, but using two leads for increased current; requires same type, connections to both, and ignores conflicts between them in ERC)