Proposed: three commands.  
Not set in stone, will likely need to be adjusted over time.
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(1) scene definition

scene scene-name (save | reset [frames[,trans-name]])
scene list
~scene (scene-name | all)

Notes: 
- "scene-name" and "trans-name" are user-specified, see #2 for the latter
- default scene not necessary
- save corresponds to scadd
- reset corresponds to scshow
- scsave to file omitted for now, could be third option "write pathname"
- alternative: expand existing reset command to restore scenes, then only need
    scene scene-name
    scene list
    ~scene (scene-name | all)

(2) transition definition

transition trans-name [color color-options] [visibility vis-options] \
  [style style-options] [global global-options]
transition list
~transition (trans-name | all)

Notes:
- keywords could be different, I just wanted to convey purpose/scope 
  ... many many possibilities, e.g. 
  color ( linear [f1,f2] | sigmoid [f1,f2] | abrupt f ) where f are % fractions
  of the transition from the preceding scene 0 and the next scene 100.
  A percentage is easier to type than values within the 0-1 range.
  ... would we want to allow different behaviors for different models?

(3) animation between scenes, analogous to existing "fly" command:
fly [master-frames]  pos1  [[ frames1_2] pos2]  \
    [[ frames2_3] pos3] ... [[ framesN-1_N] posN] 
see http://www.cgl.ucsf.edu/chimera/docs/UsersGuide/midas/fly.html

animate [master-frames[,master-trans]]  scene1 \
  [[ frames1_2[,trans1_2]] scene2] [[ frames2_3[,trans2_3]] scene3] ... \
  [[ framesN-1_N[,transN-1_N]] sceneN] 

Notes:
- scene*, trans* previously defined with scene and transition, respectively
- alternative: expand existing fly command to do all this
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Concerns:
- does this limit transitions (other than positions, which would be
splined as in "fly") to depending on only the bracketing pair of scenes? 
(Do we need fancier transitions that also depend on further preceding and 
following scenes?)
a given single transition?
- how does animation integrate with recording, 2dlabels, coordset, other
commands with frame arguments (move,turn,rock,roll)? 
Simply as interleaved commands between "movie record" and "movie stop"?
Could coordset or 2dlabels changes occur at the same time as other transitions?
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