def do_it=
 for j=0 upto n-1: jj:=width*j; jjj:=jj+width; jjjj:=j;
 scantokens("input "&filename); endfor enddef;
string filename;

def row(expr x) =
 cc:=(x-1)div height; rr:=height-1-((x-1)mod height);
 if rr=height-1: beginchar(cc*n+jjjj,width/pt,height/pt,0); fi enddef;

def cols(text t) =
 for tt:=t: exitif tt>=jjj; if tt>=jj:
  addto currentpicture also unitpixel shifted (tt,rr); fi endfor
 if rr=0: xoffset:=-jj; endchar; fi enddef;