position nodes on discrete multiples of 100

I have a few ideas for encouraging arrangements of definitions to have
more life in the sense of Christopher Alexander:
* quantize the x axis to discrete columns
* don't force a single grid; that can look boring. Each node occupies
  600px of width, so quantizing at multiples of 100 gives us 6 possible
  arrangements.
* always position new nodes within the current viewport
* always position new nodes at least some amount of padding above to
  give the node above some room to grow
* forbid overlap
  * moving a node against another should push the other node as well
  * I want a node to yield some resistance, but I don't want resistance
    to grow linearly by the number of nodes pushed

0018-on.update

@@ -19,6 +19,8 @@ on.update = function(dt)
 	if Move then
 		Move.node.x = sx(App.mouse_x()-Move.xoff)
 		Move.node.y = sy(App.mouse_y()-Move.yoff)
+		-- quantize the x axis to discrete columns
+		Move.node.x = round(Move.node.x/100)*100
 		A1(Move.node.key)
 		return
 	end

0115-round

@@ -0,0 +1,5 @@
+round = function(f)
+	-- apparently this is wrong for one floating point under 0.5
+	-- there's an alleged better way at https://stackoverflow.com/questions/18313171/lua-rounding-numbers-and-then-truncate/58411671#58411671
+	return math.floor(f+0.5)
+end