acdw.net/R

# -*- awk -*-

BEGIN {
    FS="\n"; RS=""
    i=0
    _end = "__END__"
    _el=" el "; _defel="p"
}

function start(el, not_html) { # start an element
    el = (el ? el : _defel)
    pre_el = (not_html ? "" : _el)
    split(el, el_a, " ")
    lvl[++i] = el_a[1]
    return "$(<<" _end i lvl[i] pre_el el "\n"
}

function end() { # end an element
    return "\n" _end i lvl[i--] "\n)"
}

function wrap(par) { # wrap a non-delimited paragraph with <p>
    if (par !~ /^<.*>$/)
        return start() par end()
    else
        return par
}

/^@[^@()]/ { # html element
    $1 = start(substr($1, 2))
    sub(/\n?$/, end())
    print
    next
}

/^@@[^()]/ { # arbitrary command
    $1 = start(substr($1, 3))
    sub(/\n?$/, end())
    print
    next
}

/^@\(/ { # nested element begin
    print start(substr($1, 3))
    $1=""; $0=$0
}

/^@@\(/ { # nested arbitrary command begin
    print start(substr($1, 4))
    $1=""; $0=$0
}

/@@?\)$/ { # nested element end
    nf=$NF; NF--; $0=$0; print
    if ($0) {
        print wrap($0)
        print substr(nf, 0, length(nf)-2) end()
    } else
        print end()
    next
}

{ print wrap($0); }