support for APL character overstrikes in en_US.UTF-8/Compose

[Geoff Streeter]

I have proposed some additional compositions to support the APL programming language. See https://bugs.freedesktop.org/show_bug.cgi?id=44059

APL started with IBM golf ball terminals and has always had the concept of composed characters which were built with overstrikes. So ∇ overstruck | produced ⍒.

The additions proposed cover all of the overstrikes which can be produced by the variants of apl keyboards in xkb/sybols/apl which has just been updated by Sergey.

I have also provided support for the circled alphabet and the circled digits. Since the letter "o" had already been used with "c" and "r" for the copyright  and registered symbols, I used the digit "0". I also coded the use of the "○". I have only coded these in one order the "0" or "○" have to come first.

For the actual APL characters I have coded both orders.

I could have worked my way through the whole "Mathematical Operators" and "Miscellaneous Technical" but I am not aware of the cost of an extra line in "Compose". I have only coded the characters that I am aware of being in use in extant APLs.

Is en_US.UTF-8/Compose.pre the only file I should be changing? Should I be creating an "apl" locale with its own Compose file that is then included in en_US.UTF-8/Compose?

Geoff Streeter