procedure main()
    while writes("List? ") & line := read() do
        line ?
            if sum := list(line) & pos(0) then write(line, ": sum is ", sum)
            else write(line, ": invalid")
end

procedure list()
    ="[]" & suspend 0
    ="[" & sum := values() & ="]" & suspend sum
end

procedure values()
    num := value() & ="," & sum := values() &
        suspend num + sum
    suspend value()
end

procedure value()
    suspend (list() | tab(many(&digits)))
end