Hi blockers and alek in particular, the code is cool and I like that each method is commented. I'm starting to write a real class comment that we will be able to use in a documentation. But I cannot run the example so this is difficult to write tests. So Alek how do you know that you make progress? Where should I start? I thought BlElement was the starting point. Side note: Now I wonder if we want to have all the assert: May be yes and we should have an opal plugin to turn them off after. allChildrenDo: aBlock "Evaluate aBlock for all my children gathered recursively (excluding the receiver). aBlock must not be nil. aBlock must accept exactly one argument" self assert: aBlock isNotNil. self assert: aBlock argumentCount = 1. self childrenDo: [ :child | aBlock value: child. child allChildrenDo: aBlock ] Stef