This tree structure seems similar to ikegami's code, however I haven't understood that fully yet, so I'm not sure what's the difference.

My tree is the same as your $ctree:

 A -- ...
/
\   X -- P -- ...
 \ /
  L
   \
    e -- n -- o -- c -- ...
[download]

except numbers are considered atomic in mine.

I don't bother collapsing into your $stree form:

 A -- ...
/
\   XP -- ...
 \ /
  L
   \
    enoc -- ...
[download]

You must type the marked characters, but the others are optional

The difference with mine is that I made more character mandatory. The rational is that the OP wanted to the result to resemble the original as much as possible.

Your mandatory characters:

• Ambiguous characters.

Lenoc3_duallayer_1 -> Le3d1
^^   ^ ^         ^
[download]

My mandatory characters:

• Ambiguous characters.
• A sequence of 0+ uppercase letters followed by a sequence of 0+ lowercase letters preceding an ambiguous lowercase letter.
• First and second unambiguous character of a text sequence
• Nonletters (digits, underscores)

Lenoc3_duallayer_1 -> Len3_du_1
^^^  ^^^^       ^^
[download]

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