def coerce(cls, string):
        """Create a :class:`Requirement` object from a given package spec."""
        match = re.match(cls.spec_regex, string)
        if not match:
            raise InvalidRequirementSpecError("could not parse requirement")
        package = match.group(1)
        if all(match.group(2, 3)):
            version = "".join(match.group(2, 3))
        else:
            version = None
        return cls(package, version)
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)