math-skills: add helper for pretty formulas

[Entkenntnis]

this is slightly involved as it should probably handle signs, brackets, spacings, ...

[Entkenntnis]

no general solution feasable, so stay pragmatic and add pp-helpers along the way.

Don't use clever tricks in the generator! Stay clean there and then let the helpers do the prettifying stuff.

So it's best to move on with new exercises.