Refactoring note. Placeholder.

[besok]

Placeholder to myself or somebody to find some time to refactor the module in the following points

• replace all expect with ok_or(..)
• refactor grammar (it feels like we need to squash some levels)
• rewrite ast prasing. (it feels there is the implementation that heavily relies on the knowledge of the grammar itself which can be painful to handle in some cases)
• ensure that all possible cases are covered with tests