for ($i = 0; $i < @freq; $i += 2) {
$expr .= "(\$f0 >= $freq[$i] && \$f0 <= $freq[$i+1]) ||";
my $r = Spot::ftor($freq[$i], $freq[$i+1]);
for ($i = 0; $i < @freq; $i += 2) {
$expr .= "(\$f0 >= $freq[$i] && \$f0 <= $freq[$i+1]) ||";
my $r = Spot::ftor($freq[$i], $freq[$i+1]);