Add notes for for-like loop.

README.md

@@ -39,6 +39,37 @@ Documentation
 TODO
 ----
 
+### `for`-like loop
+
+We have range-checking in the abstract analysis now, but we lack practical ways
+to use it.
+
+We can `and` a value to ensure it is within a certain range.  However, in the 6502
+ISA the only register you can `and` is `A`, while loops are done with `X` or `Y`.
+Insisting this as the way to do it would result in a lot of `TXA`s and `TAX`s.
+
+What would be better is a dedicated `for` loop, like
+
+    for x in 0 to 15 {
+        // in here, we know the range of x is exactly 0-15 inclusive
+        // also in here: we are disallowed from changing x
+    }
+
+However, this is slightly restrictive, and hides a lot.
+
+However however, options which do not hide a lot, require a lot of looking at
+(to ensure: did you increment the loop variable? only once? etc.)
+
+The leading compromise so far is an "open-faced for loop", like
+
+    ld x, 15
+    for x downto 0 {
+        // same as above
+    }
+
+This makes it a little more explicit, at least, even though the loop
+decrementation is still hidden.
+
 ### Save registers on stack
 
 This preserves them, so that, semantically, they can be used later even though they