docs: def _reserved_symbols(self):
          """
          Helper property for the build_remap_symbols method.  This
          property first resolves _all_ local references from parents,
          skipping all locally declared symbols as the goal is to generate
          a local mapping for them, but in a way not to shadow over any
          already declared symbols from parents, and also the implicit
          globals in all children.
  
          This is marked "private" as there are a number of computations
          involved, and is really meant for the build_remap_symbols to use
          for its standard flow.
          """
  
          # In practice, and as a possible optimisation, the parent's
          # remapped symbols table can be merged into this instance, but
          # this bloats memory use and cause unspecified reservations that
          # may not be applicable this or any child scope.  So for clarity
          # and purity of references made, this somewhat more involved way
          # is done instead.
          remapped_parents_symbols = {
              self.resolve(v) for v in self.non_local_symbols}
  
          return (
              # block implicit children globals.
              self.global_symbols_in_children |
              # also not any global symbols
              self.global_symbols |
              # also all remapped parent symbols referenced here
              remapped_parents_symbols
          )