In text-lines.diesel:
The text_lines data structure represents a series of lines with
newlines separating them. The new_text_lines function breaks a
string after newlines into separate text lines. The lines function
provides access to the lines of text. The indent function adds count
spaces to the front of each text line. The as_collector and
as_string functions convert the text_lines object to a
collector or a flat string representation, with embedded
newlines.