'Initializes a L{Speed} object.
@param speed: The speed that this object represents, expressed in
meters per second.
@type speed: C{float}
@raises ValueError: Raised if C{speed} is negative.'

def __init__(self, speed):

    if (speed < 0):
        raise ValueError(('negative    speed:    %r' % (speed,)))
    _BaseSpeed.__init__(self, speed)
