code: def pdf(self, phi):
          r
          if self._n is None:
              pdf = np.empty_like(phi)
              pdf.fill(.5/np.pi)
              return pdf
  
          phi = np.asarray(phi)
  
          pdf = self._pdf(phi)
          pdf /= 2.*np.pi
  
          return pdf
docs: r"""
          Evaluate the flow PDF `dN/d\phi`.
  
          :param array-like phi: Azimuthal angles.
  
          :returns: The flow PDF evaluated at ``phi``.