Update to explain the "dummy" outermost wrapper constructor

syndicate/HOWITWORKS.md

@@ -1,7 +1,7 @@
 # Efficient, Imperative Dataspaces for Conversational Concurrency
 
 Tony Garnock-Jones <tonyg@leastfixedpoint.com>  
-14 October 2018
+20 October 2018
 
 <p style="font-size:90%"><strong>Abstract.</strong> The dataspace
   model of Conversational Concurrency [is great], but implementing it
@@ -380,7 +380,7 @@ parameterized with different update procedures.
               (Handler -> [V] -> 1) ->
               1
     modify node v m_cont m_leaf m_handler =
-      walk-node node [dummy(v)]                 -- TODO: gross
+      walk-node node [outermost(v)]
       where
         walk-node :: Node -> [V] -> 1
         walk-node (cont, edges) vs =
@@ -406,6 +406,16 @@ parameterized with different update procedures.
                     let vs = projectMany v captureMap
                     m_handler handler vs
 
+The `outermost` constructor applied to `v` at the top of `modify` is
+necessary because every path in the trie structure embodied in each
+`node` is a sequence of zero or more (move, check) pairs. Each "move"
+pops zero or more items from the stack and then pushes a sub-structure
+of the topmost stack element onto the stack; the "check" then examines
+the class of the new top element, abandoning the search if it does not
+match. Without some outermost constructor, there would be no possible
+"move", and the trie would have to be expressed as a single optional
+check followed by zero or more (move, check) pairs.
+
 **Definition.** The procedure `adjustAssertion` updates the copy-count
   associated with `v` in the given index, invoking callbacks as a
   side-effect if this changes the observable contents of the
@@ -499,8 +509,6 @@ interested in the `present` constructor.
 
  - describe the cleanup function associated with a handler in the real implementation
  - figure out and describe scoped assertions / visibility-restrictions
- - check pop/index logic to ensure no off-by-ones
- - rearrange to avoid the `dummy(...)` wrappers
 
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