html_text.pl -- Switch between HTML and plain text output

Allow generating plain (colored) text from html//1 compatible calls. We do this in two steps, first creating a token list and next deal with state using fixup_layout/2. It would be nicer if we could avoid the latter step, but we need to maintain state such as indentation and attributes and we cannot pass that around in an additional argument as there are calls that do not know about this transformation in between.

The translation is highly specific for the calls done in html.pl. If you change CSS classes or the HTML DOM produced you likely have to make changes here as well.

To be done

- It might be better to maintain a stack of parent DOM nodes in a backtrackable global variable to deal with the state.

 emit_as(+HTML, +Mode)//

Causes all emit//1 calls to either behave as html//1 or convert the commands to output for print_message_lines/3.

 emitting_as(-Mode)

Current emit mode. One of plain or html

 fixup_layout(+Tokens, -Final)

Fixup layout instructions in the token list.

Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 emit(Arg1, Arg2, Arg3)