Update Jonas

Edited 1.2 -> Simplifying by pattern matching, not sure whether one can
simplify (d) any further

Edited 2.1 -> these guards seemed strange to me

Edited 2.3
Variable xs was missing

Edited 2.5.
Should work now, but not using : , using ++ instead. Not sure wether
it’s the better solution

edited 3.1
Should now work properly