DOC: What is "expression complexity"?

[MartinThoma]

I would love to see a hint in the README what expression complexity is

[flxai]

@MartinThoma Thanks for asking the question. I was wondering the same.

Reconstructing the idea from the current source code it appears to be computed on the AST. Apparently its the sum of all the AST's nodes multiplied by a custom factor per node type. But this may not be the full picture given that I've not looked at all the source's details (like score_addon etc.).

Maybe @Melevir or @vQuadX can shed some light on the details?