/*Given 2 strings, a and b, return a new string made of the first char of a and the last char of b, so "yo" and "java" yields "ya". If either string is length 0, use '@' for its missing char.

java.lang.IllegalStateException: Cannot register highlighting rule for characters at Range [from [line=25, lineOffset=17] to [line=25, lineOffset=21]] as it overlaps at least one existing rule" when ...