procedure main()
   write("Type arithmetic expressions, 1 per line, eg. a+b*(c-d)/(6+e)")
   while line := read() do
    { marker := ""
      every i := bal( '+-*/', , , line) do
         marker := left(marker, i-1) || line[i]
      write( line, "\n", marker)                # write line and marker
    }
end
# note: every, bal
# From  The Icon Prog' Lang'  1st edn., Ch 11, sec 11.6, p115