sub atan {
    return 0 if $_[0]==0;
    return $_[0] > 0 ?
         atan2(sqrt(1+$_[0]*$_[0]),sqrt(1+1/($_[0]*$_[0]))) :
        -atan2(sqrt(1+$_[0]*$_[0]),sqrt(1+1/($_[0]*$_[0])));
}