An intermediate representation of sequences of instructions for a stacked based computer is a code graph using a numbering method on the nodes of the graph, along with a set of relations among the nodes, to determine, in a single pass, the independence of each node or sub-graph represented by the node. The lead comprises a casing housing a reel, a lead windable about the reel, the casing having an outlet for passage of the lead and further comprising a moveable member for engagement and disengagement with the reel, a part of the moveable member being engageable with a retainer on movement of the member towards the reel to lock the member against the reel in a single action. The device comprises a laser, a biasing circuit for providing a DC bias current, an RF circuit providing a RF signal and a drive circuit for combining the RF signal with the DC bias current to provide a drive current to the laser. A removable insert is secured to the outer surface of each of the uprights and protects the uprights from coming in contact with another member and being worn down.