def remove(self, elem):
        """Remove a requirement.

        :param elem: a string or :class:`Requirement` instance.
        """
        if isinstance(elem, text_type):
            for requirement in self:
                if requirement.package == elem:
                    return super(Requirements, self).remove(requirement)
        return super(Requirements, self).remove(elem)
def force_fft(order, force, ax=0):
    """plot force harmonics"""
    unit = 'N'
    scale = 1
    if min(force) < -9.9e3 or max(force) > 9.9e3:
        scale = 1e-3
        unit = 'kN'
    if ax == 0:
        ax = plt.gca()
    ax.set_title('Force Harmonics / {}'.format(unit))
    ax.grid(True)
    try:
        bw = 2.5E-2*max(order)
        ax.bar(order, [scale*t for t in force], width=bw, align='center')
        ax.set_xlim(left=-bw/2)
    except ValueError:  # empty sequence
        pass