-When the program is fully parsed, we need to walk the list of variables
-to find any that weren't merged away and that aren't global, and to
-calculate the frame size and assign a frame position for each
-variable. For this we have `scope_finalize()`.
+When the program is fully parsed, each function is analysed, we need to
+walk the list of variables local to that function and assign them an
+offset in the stack frame. For this we have `scope_finalize()`.
+
+We keep the stack from dense by re-using space for between variables
+that are not in scope at the same time. The `out_scope` list is sorted
+by `scope_start` and as we process a varible, we move it to an FIFO
+stack. For each variable we consider, we first discard any from the
+stack anything that went out of scope before the new variable came in.
+Then we place the new variable just after the one at the top of the
+stack.