Vector rejection.

    Calculated by subtracting from `vec` the projection of `vec` onto
    `vec_onto`:

    .. math::

        \\mathsf{vec} - \\mathrm{proj}\\left(\\mathsf{vec},
        \\ \\mathsf{vec\\_onto}\\right)

    Parameters
    ----------
    vec
        length-R |npfloat_| --
        Vector to reject

    vec_onto
        length-R |npfloat_| --
        Vector onto which `vec` is to be rejected

    Returns
    -------
    rej_vec
        length-R |npfloat_| --
        Rejection of `vec` onto `vec_onto`

def rej(vec, vec_onto):
    """ Vector rejection.

    Calculated by subtracting from `vec` the projection of `vec` onto
    `vec_onto`:

    .. math::

        \\mathsf{vec} - \\mathrm{proj}\\left(\\mathsf{vec},
        \\ \\mathsf{vec\\_onto}\\right)

    Parameters
    ----------
    vec
        length-R |npfloat_| --
        Vector to reject

    vec_onto
        length-R |npfloat_| --
        Vector onto which `vec` is to be rejected

    Returns
    -------
    rej_vec
        length-R |npfloat_| --
        Rejection of `vec` onto `vec_onto`

    """

    # Imports
    import numpy as np

    # Calculate and return.
    rej_vec = vec - proj(vec, vec_onto)
    return rej_vec