'Define the addition for Dimension.
Addition of dimension has a sense only if the second object is
the same dimension (we don\'t add length to time).'

def __add__(self, other):

    if (not isinstance(other, Dimension)):
        raise TypeError(("Only    dimension    can    be    added;    '%s'    is    not    valid" % type(other)))
    elif (isinstance(other, Dimension) and (self != other)):
        raise ValueError(("Only    dimension    which    are    equal    can    be    added;    '%s'    and    '%s'    are    different" % (self, other)))
    return self
