Since m4 was linked in https://textboard.org/prog/538#t538p18 here is the Sierpinski carpet >>17 in m4:
$ cat carpet.m4
divert(-1)
define(`carpet_level',3)
define(`carpet_empty',` ')
define(`carpet_full',`m4')
define(`carpet_size',eval(3**carpet_level))
define(`carpet_pred',`ifelse(eval(($1==0)||($2==0)),1,1,`ifelse(eval((($1%3)==1)&&(($2%3)==1)),1,0,`carpet_pred(eval($1/3),eval($2/3))')')')
define(`carpet_symbol',`ifelse(carpet_pred($1,$2),1,carpet_full,carpet_empty)')
define(`carpet_xjoin',`ifelse($1,1,`$2(0)`$4'',`carpet_xjoin(decr($1),`$2',`$3',`$3'$2(decr($1))`$4')')')
define(`carpet_linesymbol',`carpet_symbol(carpet_currentline,$1)')
define(`carpet_line',`pushdef(`carpet_currentline',$1)carpet_xjoin(carpet_size,`carpet_linesymbol',,)popdef(`carpet_currentline')')
define(`carpet_carpet',`carpet_xjoin(carpet_size,`carpet_line',`
',)')
divert(0)carpet_carpet
$ m4 carpet.m4
m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4
m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4
m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4
m4m4m4 m4m4m4m4m4m4 m4m4m4m4m4m4 m4m4m4
m4 m4 m4 m4m4 m4 m4 m4m4 m4 m4 m4
m4m4m4 m4m4m4m4m4m4 m4m4m4m4m4m4 m4m4m4
m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4
m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4
m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4
m4m4m4m4m4m4m4m4m4 m4m4m4m4m4m4m4m4m4
m4 m4m4 m4m4 m4 m4 m4m4 m4m4 m4
m4m4m4m4m4m4m4m4m4 m4m4m4m4m4m4m4m4m4
m4m4m4 m4m4m4 m4m4m4 m4m4m4
m4 m4 m4 m4 m4 m4 m4 m4
m4m4m4 m4m4m4 m4m4m4 m4m4m4
m4m4m4m4m4m4m4m4m4 m4m4m4m4m4m4m4m4m4
m4 m4m4 m4m4 m4 m4 m4m4 m4m4 m4
m4m4m4m4m4m4m4m4m4 m4m4m4m4m4m4m4m4m4
m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4
m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4
m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4
m4m4m4 m4m4m4m4m4m4 m4m4m4m4m4m4 m4m4m4
m4 m4 m4 m4m4 m4 m4 m4m4 m4 m4 m4
m4m4m4 m4m4m4m4m4m4 m4m4m4m4m4m4 m4m4m4
m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4
m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4m4 m4
m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4m4