pep-550: A couple of minor additions to the impl strategy section (#345)

pep-0550.rst

@@ -911,6 +911,11 @@ complexity:
   ``N`` is the total number of items in the current **execution**
   context.
 
+Essentially, using HAMT for Local Contexts instead of Python dicts,
+allows to bring down the complexity of ``sys.get_execution_context()``
+from O(N) to O(log\ :sub:`32`\ N) because of the more efficient
+merge algorithm.
+
 
 Approach #3: Use HAMT and Immutable Linked List
 -----------------------------------------------
@@ -930,7 +935,7 @@ copied by reference in ``sys.get_execution_context()`` (eliminating
 the expensive "merge" operation.)
 
 With this approach, ``sys.get_execution_context()`` will be an
-O(1) operation.
+**O(1) operation**.
 
 
 Summary