Errors and exit

Report on stderr, exit with a status.

main (os, print)

parse(text str) int ? str
    if len(text) == 0
        "empty input"
    len(text)

main()
    when parse("np") is
        n print.line("{n}")
        problem
            # print.error writes to stderr; os.exit sets the status.
            shown = print.error("error: {problem}")
            os.exit(2)