\catcode`\@=11
\newif\ifII@itis      %% reused from \ifisint %%
\def\II@rmsign #1{\ifx#1+\else\ifx#1-\else    %%
 \II@endrm#1\fi\fi\II@rmsign}%  remove signs: %%
\def\II@endrm #1\fi\fi#2{\fi\fi#1}% `+' & `-' %%
\let\Boolend=\iffalse \font\II@font=cmr10 %   %%
\let\IIcurrentmode=\errorstopmode % CONFIGURE %%
\def\II@W{W}\def\II@octW{'W}\def\II@hexW{"W}% %%
%% declarations
\newdimen\II@frac
\countdef\II@cnt=255 \dimendef\II@dim=255
%% helper macros; some use \ifisint's sentinel W
\def\II@bad #1#2#3#4#5#6\II@end{%   numeric part
 \def\II@id{#1W}% is missing but maybe with unit
 \edef\II@X{#6}\ifx\II@X\empty
  \edef\II@X{#5}\ifx\II@X\empty\else\II@Bad\fi
 \else \edef\II@X{\II@mklc#2#3#4W}%
  \ifx\II@X\II@rueW
  \else\ifx\II@X\II@truW\II@Bad
  \else \II@itistrue
 \fi\fi\fi}
\def\II@rueW{rueW}\def\II@truW{truW}
\def\II@Bad{\ifx\II@id\II@W
 \else\ifx\II@id\II@octW
 \else\ifx\II@id\II@hexW
 \else \II@itistrue
 \fi\fi\fi}
\def\II@mklc #1{\if#1pp\else\if#1Pp\else
 \if#1tt\else\if#1Tt\else
 \if#1bb\else\if#1Bb\else
 \if#1ss\else\if#1Ss\else
 \if#1rr\else\if#1Rr\else
 \if#1uu\else\if#1Uu\else
 \if#1ee\else\if#1Ee\else
  \II@endlc#1\fi\fi\fi\fi\fi\fi\fi\fi\fi\fi\fi
 \fi\fi\fi \II@mklc}%  `W' and `m' stop \II@mklc
\def\II@endlc #1\fi\fi\fi\fi\fi\fi\fi\fi\fi\fi
 \fi\fi\fi\fi#2{\fi\fi\fi\fi\fi\fi\fi\fi\fi\fi
 \fi\fi\fi\fi#1}
\def\II@getfrac #1mm\II@end{\global\II@frac=0#1}
\def\II@getcalc{%\II@calc=coerced\II@int\II@frac
 \ifdim\II@unit=26.11119pt %  \II@unit is ``pt''
  \II@dim=\ifnum\II@int<16384
   \II@int\else 0\fi pt
 \else\ifdim\II@unit=27.77786pt %   it is ``bp''
  \II@dim=\ifnum\II@int<16323
   \II@int\else 0\fi bp
 \else\ifdim\II@unit=26.16673pt %   it is ``sp''
  \II@dim=\ifnum\II@int<1073741824
   \II@int\else 0\fi sp
 \else \II@dim=0pt \fi\fi\fi
 \II@cnt=\II@dim \advance\II@cnt by \II@frac
 \edef\II@calc{\number\II@cnt}}
\def\II@point #1#2\II@end{% assign digits of the
 \afterassignment\II@mklc  % fraction to \II@cnt
 \ifx#1.\II@cnt=0#2%
 \else\ifx#1,\II@cnt=0#2%
 \else \II@cnt=0#1#2%
 \fi\fi}
\def\II@getunit #1{\afterassignment\II@hdlfrac
 \II@cnt=#1\relax}
\def\II@rmtrue{\ifdim\wd0>40pt \the\II@dim
 \else \the\wd0 \fi}
%% main macro
\def\ifisdim #1\Boolend{\II@itisfalse     % S1.3
 \edef\II@dist{\II@rmsign#1mm}%             S1.1
 \edef\II@dist{\expandafter\II@rmsign\II@dist}%
 \expandafter\II@bad
  \II@dist\empty\empty\empty\empty\II@end % S1.2
 \ifII@itis                         % S4.1, S4.4
  \wlog{=== start ignore}\batchmode\begingroup
   \setbox0=\hbox{\II@font
    \afterassignment\II@getfrac
     \II@cnt=\II@dist\II@end              % S2.2
    \xdef\II@int{\the\II@cnt}}%             S2.1
   \setbox0=\hbox{\II@font
    \afterassignment\II@point
     \II@cnt=\II@dist\II@end}\II@dim=\wd0
   \advance\II@dim by -17.80559pt % width `true'
   \xdef\II@unit{\II@rmtrue}%               S2.3
   \setbox0=\hbox{\II@font\II@dim=#1mm%     S3.1
    \xdef\II@val{\ifdim\II@dim<0pt-\fi
     \the\II@dim}}%
   \xdef\II@wd{\the\wd0}%
  \endgroup\IIcurrentmode\wlog{=== stop ignore}%
  \ifdim\II@wd=16.66672pt % width ``mm''    S3.2
   \ifdim\II@val=\maxdimen \II@getcalc
    \ifnum\II@calc=1073741823 %             S4.2
    \else \II@itisfalse                   % S4.3
   \fi\fi
  \else \II@itisfalse                     % S3.3
 \fi\fi \ifII@itis}
\catcode`\@=12