fun water_vol (wh: int list) =
    let fun count_vol max hs acc = 
	    case hs of
		[] => acc
	      | h2::hs' => if (max - h2) > 0 andalso (List.exists (fn x => x>max) hs')
			   then count_vol max hs' (acc+max-h2)
			   else count_vol h2 hs' acc
    in
	count_vol (hd wh) (tl wh) 0
    end