html.parser.printer: reasonable tree-structured formatting with prettyprint-html
html.parser.printer: explaining comment
html.parser.printer: unit tests
html.parser.printer: remove printer symbol
html.parser.printer: fixed to use repeated write calls instead of surround
html.parser.printer: some fixes.

Fix html-text. (may not be perfect, but "works" now)
Don't decrement indentations if we didn't increment them.
sequences: don't allow negative repetitions.
html.parser.printer: prevent negative indentations.
sequences: add some tests for repetition.