A formalisation of design methods - a lambda-calculus approach to systems design with an application to text editing

ion functiontest if position is in gapabstraction functionabstraction functionabstraction functionabstraction functionabstraction functionabstraction functionabstraction functionabstraction functionabstraction functionstrengthened text-invariant next position (rightwards)end-of-buffer predicateend-of-line predicatedot is at end-of-bufferdot is at end-of-linenext position (Ieftwards)beginning-of-buffer predicatedot is at beginning-of-bufferbeginning-of-line predicatedot is at beginning-of-line strengthened window-invariantinvariant of wi_packageinitialisation procedurerestore WI ( text modified)restore WI (dot modified)arbitrary dot modification buffer is ready for insertmake buffer readyauxiliary for make_ready