XT gyoretsu - extended ncurses version of "Renshu-cho gyoretsu"

Goal of the game is to clear entire field by matching pairs of
neighbouring (adjacent or on the same line/column without any other
numbers in between) numbers, which must be either equal, or their
sum must be equal to strike out sum.

WWW: https://github.com/Penguinum/xtgyoretsu-forked/
WWW: http://saahriktu.org/games.html