Public: Treeprocessors are run on the Document after the source has been parsed into an abstract syntax tree, as represented by the Document object and its child Node objects.
Asciidoctor invokes the process method on an instance of each registered Treeprocessor.
QUESTION should the treeprocessor get invoked after parse header too?
Treeprocessors must extend Asciidoctor::Extensions::Treeprocessor.
# File lib/asciidoctor/extensions.rb, line 319 def process end