Don't divide by 0 for 0-length edges
Things overlap if their cores overlap for minor (~10%) speedup
Better computation of neighbors for speed and more betterness
Dead critters can't do much
Getting damaged is an event
Pull collision handling out of run()
Move stipple function to the top level
Graphical representation of secretion trails

To be useful, this needs:

    * A secretion class, with compound id
    * Some indication (a sound-like text?) of the compound
    * Control (via an action) of what's to be secreted
    * A mid-granularity grid hash to facilitate finding near-by secretions
    * Merging of adjacent secretion bubbles for the same compound
    * Tie in with the sense system
Refractor shapes to put 'front' at the start
Move collision/impact management up to the world (and give everything mass)
Raise distance_to to PhysicalObjects
Allow worlds with no critters
Blocks
Hardness
Accept list of brains files on command line
Gltich suppression
In a world with pits and pushing, any PhysicalObject can die
Variable block size and mass
First step towards reifying secretions
Put secretions under critter's control