\magnification=4000	% specify LARGE proofs
\nopagenumbers		% turn off page numbering
\raggedbottom		% not to worry about bottom lines
\font\io=io		% load the new font
\io			% select the new font
\def\test#1{		% define a test macro
  \chardef\I=#1		% the character number of I
  \chardef\O=79		% the character number of O
  \leftline{\O\I\O\O\I\I\O\O\O\I\I\I\O\I} % the test line
  }			% end of the \def of \test
\newcount\code		% introduce a count register
\loop\advance\code by 1 % run through the values 1,2,3...
  \test\code		% set a test line
  \ifnum\code<16\repeat % repeat until \code=16
\bye			% quit