A group of lines of a code element
Expresso.Element.Code.build/1 makes one group for each item of the
reveal option, with the specification [from: :next]. The groups are the
children of the code element, so the transformer gives each group its steps,
and the verifier checks each group against the code element. A group is not
an entity of the DSL.
A token group of the highlight option has the text of its tokens in
each, and no line numbers.