anchor.diff:  room to maneuver without having to flip to the opposite end of the pan
   range regardless of how the fixture has been hung.
   
  +If the fixture cannot pan far enough to aim directly at the audience
  +when it is hung in its default orientation, you may be better off
  +choosing a different default hanging orientation. But if you do not
  +want to do that, you can set this to the closest value outside the
  +legal DMX range which would cause the fixture to pan that far if it
  +were legal and possible, and Afterglow will still be able to figure
  +out and use the legal movements that the fixture is capable of.
  +
   |`:pan-half-circle`
   
  -|.
  +|If this fixture is a moving head capable of pan movements, this entry
  + tells Afterglow the amount it needs to add to the DMX value sent on
  + the fixture's Pan channel to pan it halfway around a circle in a
  + counterclockwise direction. Afterglow uses this to figure out how to
  + aim the head exactly where you want it. If your fixture is not
  + capable of panning that far, this value may be larger than a legal
  + DMX value. That is fine, Afterglow will figure that out. Simply
  + always give it the value which, when added to some legal Pan channel
  + value, would cause the fixture to rotate counterclockwise halfway
  + around a circle if it could rotate that far. (This number could be
  + negative if the fixture turns clockwise when the pan value is
  + increased in its default hanging orientation.)
  +
  +The <<show_space#show-space,Show Space>> page explains how to figure
  +out which rotations are clockwise or counterclockwise with respect to
  +different axes. Pan motions are rotations around the fixture Y axis.
   
   |`:tilt-center`
   
  -|.
  +|If this fixture is a moving head capable of tilt movements, this
  + entry tells afterglow the DMX value to send the fixture to tilt it
  + directly at the audience when the fixture is hung at its standard
  + orientation. (The documentation you create for your fixture
  + definition needs to explain what this default orientation is, so that
  + people patching your fixture can figure out the proper angle
  + information to tell Afterglow if they hung it in a different
  + orientation, as explained in <<show_space#show-space,Show Space>>. The
  + `:tilt-center` value should tilt the light so it is aimed exactly
  + along the show Z axis when also panned to `:pan-center`.
  +
  +Some fixtures can tilt more than once around a full circle, so you may
  +have a choice of values to supply here, all of which tilt the fixture
  +directly towards the audience in your default hanging orientation. If
  +so, pick one towards the middle of the DMX range, giving Afterglow
  +room to maneuver without having to flip to the opposite end of the tilt
  +range regardless of how the fixture has been hung.
  +
  +If the fixture cannot tilt far enough to aim directly at the audience
  +when it is hung in its default orientation, you may be better off
  +choosing a different default hanging orientation. But if you do not
  +want to do that, you can set this to the closest value outside the
  +legal DMX range which would cause the fixture to tilt that far if it
  +were legal and possible, and Afterglow will still be able to figure
  +out and use the legal movements that the fixture is capable of.
   
   |`:tilt-half-circle`
   
  -|.
  +|If this fixture is a moving head capable of tilt movements, this entry
  + tells Afterglow the amount it needs to add to the DMX value sent on
  + the fixture's Tilt channel to tilt it halfway around a circle in a
  + counterclockwise direction. Afterglow uses this to figure out how to
  + aim the head exactly where you want it. If your fixture is not
  + capable of tilting that far, this value may be larger than a legal
  + DMX value. That is fine, Afterglow will figure that out. Simply
  + always give it the value which, when added to some legal Tilt channel
  + value, would cause the fixture to rotate counterclockwise halfway
  + around a circle if it could rotate that far. (This number could be
  + negative if the fixture turns clockwise when the tilt value is
  + increased in its default hanging orientation.)
  +
  +The <<show_space#show-space,Show Space>> page explains how to figure
  +out which rotations are clockwise or counterclockwise with respect to
  +different axes. Tilt motions are rotations around the fixture X axis.
   
   |===
   
  -== Channel Specifications
  -
  -== Head Specifications
  +=== Channel Specifications
  +
  +=== Head Specifications
   
   TODO: Finish this page.
   
  
positive.code:  explain what this default orientation is, so that people patching
   your fixture can figure out the proper angle information to tell
   Afterglow if they hung it in a different orientation, as explained in
   <<show_space#show-space,Show Space>>.
  
  Many fixtures can pan more than once around a full circle, so you may
  have a choice of values to supply here, all of which pan the fixture
  
negative.code:  was created. As desrcibed above, some fixtures can be configured to
   have different &ldquo;personalities&rdquo; which use a different
   number of DMX channels and provide a different set of features. Their
   fixture definition functions will use a `mode` argument to determine
   the mode in which the fixture is operating, and return an appropriate
   map. That map will include the chosen mode keyword as the value at
   this key.
  