anchor.diff:       {"maps", undefined, maps, "\n"
         "       Generate code that will accept and produce maps instead of\n"
         "       records.\n"},
  +     {"maps_unset_optional", {omitted, present_undefined}, maps_unset_optional,
  +      "\n"
  +      "       Specifies the interal format for optional fields\n"
  +      "       that are unset.\n"},
        {"Werror",undefined, warnings_as_errors, "\n"
         "       Treat warnings as errors\n"},
        {"W1", undefined, report_warnings, "\n"
  
positive.code: %%   <dd>Generate code that will accept and produce maps instead of
  %%       records. No .hrl file will be generated. See the `maps' option
  %%       for the function {@link file/2} for more info.</dd>
  %%   <dt>`-Werror', `-W1', `-W0', `-W', `-Wall'</dt>
  %%   <dd>`-Werror' means treat warnings as errors<br></br>
  %%       `-W1' enables warnings, `-W0' disables warnings.<br></br>
  
negative.code: %% in which case they will be proplists instead. This option is not
  %% compatible with the `nif' option.
  %%
  %% For maps, for optional fields, if not set, the
  %% `maps_unset_optional' option specifies the Erlang-internal
  %% representation; both how it is expected to be found at encoding,
  %% and how decoding will return it:
  %% <dl>
  %%   <dt>`omitted'</dt>
  %%   <dd>This means it is not included in the map</dd>
  %%   <dt>`present_undefined'</dt>
  %%   <dd>This means it is present and has the value `undefined'.
  %%       This is the default, for historical reasons mostly, due to
  %%       the way records works, but there are some caveats, although
  %%       apparently rare, but still.  imagine an enum with a symbol
  %%       `undefined', and an optional field of that enum type. It will
  %%       not be possible to tell the difference between it being unset
  %%       and being present and set.  Encoding will assume it is unset.
  %%   </dd>
  %% </dl>
  %%
  %%
  %% The `nif' option will cause the compiler to generate nif C++ code
  %% for encoding and decoding. The generated nif C++ code can be linked
  %% with the Google protobuf C++ library.  Read the file
  