bug#19145: 24.4; prettify-symbols-mode inconsistent behavior

[Ken Mankoff <mankoff@gmail.com>]

I'm having issues with the new prettify-symbols-mode. I'm not sure if
this is a bug, or just a fact that the implementation is limited, in
which case this is a feature request for a more complete implementation.

Some symbols are sometimes not being treated correctly depending on what
characters follow. For example, I have the following setup for coding
Python:

    (prettify-symbols-mode t)
    (global-prettify-symbols-mode t)
    (add-hook 'python-mode-hook
              (lambda ()
                (push '("**2" . ?²) prettify-symbols-alist)
                (push '("_x" . ?ᵪ) prettify-symbols-alist)
                (push '("delta" . ?δ) prettify-symbols-alist)))


The issue may be somewhat subjective. For example, should foo_xx appear
with a subscript x and then a normal x? Or should it appear as I assume
you are reading it with no prettification? I would argue for the latter.
What about foo_x+2? Regardless, I've created a matrix of
prettifications, what I'd expect, and what happens.

| Characters | Expected               | Actual                        | Good? |
|------------+------------------------+-------------------------------+-------|
| foo_x      | subscript x            | subscript x                   | Y     |
| foo**2     | superscript 2          | superscript 2                 | Y     |
| delta      | delta symbol           | delta symbol                  | Y     |
| foo_x+     | subscript x            | No subscript                  | N     |
| foo_xi     | no subscript           | subscript                     | N     |
| foo_x[42]  | subscript              | subscript                     | Y     |
| foo_x**2   | subscript, superscript | no subscript, yes superscript | N     |
| foo**200   | no superscript         | superscript 2                 | N     |
| delta(42)  | delta symbol(42)       | symbol                        | Y     |
| delta+42   | symbol                 | symbol                        | Y     |
| delta**2   | symbol, superscript    | symbol, superscritp           | Y     |
| delta_x    | symbol, subscript      | no symbol                     | N     |

There are some inconsistencies, like why _x+ loses prettification, but
delta+ retains it, or why foo_x_x_x works, but delta_x does not.

Thanks,

  -k.