/^<<.+>>=$/ {
    name = substr($0, 3, length($0) - 5)
    while (getline > 0) {
        if (length($0) == 0) next
        chunk[name, ++count[name]] = $0 } }
END { tangle("*", ""); printf "\n" }
function tangle(name, prefix,    i, tag, suffix) {
    for (i = 1; i <= count[name]; i++) {
        if (i == 2) gsub(/[^ \t]/, " ", prefix)
        if (match(chunk[name,i], /<<.+>>/)) {
            tag = substr(chunk[name,i], RSTART + 2, RLENGTH - 4)
            if (tag in count) {
                suffix = substr(chunk[name,i], RSTART + RLENGTH)
                tangle(tag, prefix substr(chunk[name,i], 1, RSTART - 1))
                printf "%s", suffix }
            else printf "%s%s", prefix, chunk[name,i] }
        else printf "%s%s", prefix, chunk[name,i]
        if (i < count[name]) printf "\n" } }