<b_jonas> learn ayacc is ais523\'s yacc parser generator implementation, get it from (darcs clone http://nethack4.org/projects/ayacc) or a snapshot from "http://nethack4.org/pastebin/ayacc-2020-04.tar.gz".

|a+b@| = { √(a²-b²) if a²-b² ≥ 0 ; i√(a²-b²) if a²-b² < 0 }