anchor.diff:  %%   <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>`-maps_unset_optional omitted | present_undefined'</dt>
  +%%   <dd>Specifies the interal format for optional fields that are unset.</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>
  
positive.code: %%                   {defs_as_proplists, boolean()} | defs_as_proplists |
  %%                   {descriptor,boolean()} | descriptor |
  %%                   {maps,boolean()} | maps |
  %%                   {nif,boolean()} | nif |
  %%                   {load_nif, LoadNif} |
  %%                   {i, directory()} |
  
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
  