introduce tags, structs, and reticles

this was meant to be split into smaller changes, but I realised I edited my existing revision too late.

docs/rkgk.dj

@@ -27,7 +27,8 @@ In case you edited anything in the input box on the right, paste the following t
 -- Try playing around with the numbers,
 -- and see what happens!
 
-stroke 8 #000 (vec 0 0)
+withDotter \d ->
+  stroke 8 #000 (d To)
 ```
 
 rakugaki is a drawing program for digital scribbles and other pieces of art.
@@ -83,10 +84,11 @@ If you want to draw multiple scribbles, you can wrap them into a list, which we
 
 ```haku
 -- Draw two colorful dots instead of one!
-[
-  stroke 8 #F00 (vec 4 0)
-  stroke 8 #00F (vec (-4) 0))
-]
+withDotter \d ->
+  [
+    stroke 8 #F00 (vec ((vecX (d To)) + 4) (vecY (d To)))
+    stroke 8 #00F (vec ((vecX (d To)) - 4) (vecY (d To)))
+  ]
 ```
 
 ::: aside
@@ -103,25 +105,29 @@ And what's even crazier is that you can compose lists _further_---you can make a
 It'll draw the first inner list, which contains two scribbles, and then it'll draw the second inner list, which contains two scribbles.
 
 ```haku
-[
+withDotter \d ->
   [
-    stroke 8 #F00 (vec 4 (-4))
-    stroke 8 #00F (vec (-4) (-4))
+    [
+      stroke 8 #F00 (vec ((vecX (d To)) + 4) (vecY (d To)))
+      stroke 8 #00F (vec ((vecX (d To)) - 4) (vecY (d To)))
+    ]
+    [
+      stroke 8 #FF0 (vec (vecX (d To)) ((vecY (d To)) + 4))
+      stroke 8 #0FF (vec (vecX (d To)) ((vecY (d To)) - 4))
+    ]
   ]
-  [
-    stroke 8 #FF0 (vec 4 4)
-    stroke 8 #0FF (vec (-4) 4)
-  ]
-]
 ```
 
 ::: aside
 
-Another weird thing: when negating a number, you have to put it in parentheses.
+I know this example is kind of horrendous to read right now.
 
-This is because haku does not see your spaces---`vec -4`, `vec - 4`, and `vec-4` all mean the same thing!
-In this case, it will always choose the 2nd interpretation---vec minus four.
-So to make it interpret our minus four as, well, _minus four_, we need to enclose it in parentheses.
+This is because haku currently does not have vector math, which means we have to disassemble and reassemble vectors manually.
+
+{% Another weird thing: when negating a number, you have to put it in parentheses. %}
+{% This is because haku does not see your spaces---`vec -4`, `vec - 4`, and `vec-4` all mean the same thing! %}
+{% In this case, it will always choose the 2nd interpretation---vec minus four. %}
+{% So to make it interpret our minus four as, well, _minus four_, we need to enclose it in parentheses. %}
 
 :::
 
@@ -142,15 +148,24 @@ Anyways!
 Recall that super simple brush from before...
 
 ```haku
-stroke 8 #000 (vec 0 0)
+withDotter \d ->
+  stroke 8 #000 (d To)
 ```
 
-This reads as "a stroke that's 8 pixels wide, has the color `#000`, and is drawn at the point `(0, 0)` relative to the mouse cursor."
+This reads as "given a dotter, output a stroke that's 8 pixels wide, has the color `#000`, and is drawn at the dotter's `To` coordinates."
 
 All these symbols are very meaningful to haku.
 If you reorder or remove any one of them, your brush isn't going to work!
 
-- Reading from left to right, we start with `stroke`.\
+- Reading from left to right, we start with `withDotter`.
+  We can't draw without knowing _where_ to draw, and the `withDotter` incantation lets us ask the UI for that.
+
+  - Then, `\d ->` lets us name the data we get back from the UI.
+    `d` ends up containing a few useful properties, but the most useful one for us is `To`, which contains the current mouse position (where *`To`* draw).
+
+  - We'll get to what all these sigils mean to haku later!
+
+- On the next line we have a `stroke`.
   `stroke` is a _function_---a recipe for producing data!\
   haku has [many such built-in recipes](/docs/system.html).
   `stroke` is one of them.
@@ -192,7 +207,8 @@ Note how it's parenthesized though---recall that function arguments are separate
 And with all that, we let haku mix all the ingredients together, and get a black dot under the cursor.
 
 ```haku
-stroke 8 #000 (vec 0 0)
+withDotter \d ->
+  stroke 8 #000 (d To)
 ```
 
 Nice!
@@ -206,8 +222,16 @@ So to spice things up, haku has a few shapes you can choose from!
 Recall that 3rd argument to `stroke`.
 We can actually pass any arbitrary shape to it, and haku will outline it for us.
 
-Right now haku supports two additional shapes: rectangles and circles.
-You can try them out by playing with this brush!
+The experience of drawing with that example brush is pretty crap, because it can draw dots that don't connect with each other at all.
+Let's fix that by drawing a `line` instead!
+
+```haku
+withDotter \d ->
+  stroke 8 #000 (line (d From) (d To))
+```
+
+We replace the singular position `d To` with a `line`. `line` expects two arguments, which are vectors defining the line's start and end points.
+For the starting position we use a _different_ property of `d`, which is `From`---this is the _previous_ value of `To`, which allows us to draw a continuous line.
 
 ```haku
 [