use strict;

sub f {
  my ($a,$b,$m,$n) = @_;
  my (@r) = ();
 
  my $l = " $a! $b";
  while($l =~ s/(.*?)(( \w+){$m,$n} )(.*!.*\2.*)/ $1 =$4/) {
    push @r, $2;
  };
  @r
};