r"""
    Implements Euler's formula
    :math:`\text{cis}(x) = e^{i \pi x} = \cos(x) + i \sin(x)`

def cis(x: float) -> complex:
    r"""
    Implements Euler's formula
    :math:`\text{cis}(x) = e^{i \pi x} = \cos(x) + i \sin(x)`
    """
    return np.cos(x) + 1.0j * np.sin(x)