Reference charts used to confirm operation of modal_map (which is
proving ever so tricky to get just right for ascending vs.
descending and so forth). Gist of construction is to fill in the
pitch numbers for the input scale, then the Diatonics and
chromatics, then line up the corresponding Diatonics of the output
scale with the Diatonics of the input scale, and then fill in what
chromatics are possible in the output, 'x' otherwise, then fill in
the output pitch numbers as possible.

Hmm, do not use DSC at all if the DSC does not differ? That would lead
to a this-or-that mode of algorithm... (unless can use DSC in both cases
without creating problems)

* Major to Major, link on MIDI pitch 72, parallel motion (transpose of
  0; a real example would doubtless have a non-zero transpose as
  otherwise this would be an expensive way to gain no new information).

     2      2    1   2     2     2    1   2     2    1   2     2     2    1  
  C, c#  D d#  E  F f#  G g#  A a#  B  C c#  D d#  E  F f#  G g#  A a#  B  C'
  60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84
                                  ... 72 73 ...
  C, c#  D d#  E  F f#  G g#  A a#  B  C c#  D d#  E  F f#  G g#  A a#  B  C'
     2      2    1   2     2     2    1   2     2    1   2     2     2    1  

* Major to Major, link on MIDI pitch 72, contrary motion.

     2      2    1   2     2     2    1   2     2    1   2     2     2    1  
  C, c#  D d#  E  F f#  G g#  A a#  B  C c#  D d#  E  F f#  G g#  A a#  B  C'
  60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84
  84  x 83 82 81 79 78 77  x 76 75 74 72  x 71 70 69 67 66 65  x 64 63 62 60
  C'  x  B a#  A  G f#  F  x  E d#  D  C  x  B a#  A  G f#  F  x  E d#  D  C,
      1     2    2   2     1     2    2   1     2    2   2     1     2    2  

  DSC or ASC should play out the same way, as DSC no different pattern.

* Melodic Minor to Melodic Minor, link on pitch 72 - ASC to DSC (rising
  phrases) for contrary motion. Needs to use DSC scale downwards, as
  contrary motion, so anything going up will imply something going
  down, which for melodic minor means using the descending version of
  that scale.

  C, c#  D Eb  e  F f#  G g#  A a#  B  C c#  D Eb  e  F f#  G g#  A a#  B  C'
  60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84
  84 83 82 80  x 79 78 77 76 75  x 74 72 71 70 68  x 67 66 65 64 63  x 62 60
  C'  b Bb Ab  x  G f#  F  e Eb  x  D  C  b Bb Ab  x  G f#  F  e Eb  x  D  C,

* Melodic Minor to Melodic Minor, link on 72 - DSC to ASC (falling
  phrases) for contrary motion. Descending means the ascending version
  of the scale must be used in corresponding contrary motion.

  C, c#  D Eb  e  F f#  G Ab  a Bb  b  C c#  D Eb  e  F f#  G Ab  a Bb  b  C'
  60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84
  84  x 83 81 80 79 78 77 75  x 74 73 72  x 71 69 68 67 66 65 63  x 62 61 60
  C'  x  B  A g#  G f#  F Eb  x  D c#  C  x  B  A g#  G f#  F Eb  x  D c#  C
