def add(self, elem, replace=False):
        """Add a requirement.

        :param elem: a string or :class:`Requirement` instance.
        :param replace: if `True`, packages in the set with the same name will
            be removed first.
        """
        if isinstance(elem, text_type):
            elem = Requirement.coerce(elem)
        if replace and elem in self:
            self.remove(elem)
        super(Requirements, self).add(elem)
def force(title, pos, force, xlabel='', ax=0):
    """plot force vs position"""
    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('{} / {}'.format(title, unit))
    ax.grid(True)
    ax.plot(pos, [scale*f for f in force])
    if xlabel:
        ax.set_xlabel(xlabel)
    if min(force) > 0:
        ax.set_ylim(bottom=0)