sub integrate(&@)
{
    local $x;
    my $sum;
    my $f = shift;
    my %o = (from => 0, by => 0.01, @_);
    for ($o{from} / $o{by} .. $o{to} / $o{by}) {
        $x = $_ * $o{by};
        $sum += &$f * $o{by};
    }
    $sum;
}

#> integrate { $x } from => 0, to => 1
## 0.505