pillar-markup
Microdown parse: '## Foo **BAR**'
(Microdown parse: '## Foo **BAR**') children first header will bark because
It seems like the idea of header is originally just a link used to structure a ### as substructure of ##. Hence the two functions MicSystemBlock >> header is the original, and MicHeaderBlock >> header is actually the a completely different method intended to return the parse of header block.
Besides, bodyString is a very specialized method:

So, to fix this issue, a short explanation of what is actually the wanted response.
It is breaking because bold does not understand bodyString.
header
"I am used to get an adhoc string for the header"
^ String streamContents: [ :s |
children do: [ :e | s nextPutAll: e bodyString ] ]
The API between element and their strings representation as well as the protocol to interact with element to get their string should be really revisited. But I have to concentrate for this.