my $letter='a';
if (($cnd_a + $cnd_y) <= $incrsdel){
  $aob[$yb] =~ tr/a/y/;
  $cnd_y += $cnd_a;
  $letter='c';
}
while ($cnd_y < $incrsdel){
  my $xda = int rand( $chntot + 1 );
  if ( substr( $aod[$yb], $xda, 1 ) eq $letter ) {
     substr $aod[$yb], $xda, 1, 'y';
     $cnd_y++;
  }
}