Add vim modelines.
Vertically center the register output port.
Add a "sel" point for the selector to the multiplexers.
Make multiplexers a bit smaller.
Add multiplexer metapost object.
Add a midpoint Metapost macro.
Add a hack to make \overrightarrow work in MetaPost.
Define a custom "Register" MetaObj object.
Set up MetaPost and MetaObj for drawing diagrams.