def _v3_dot_   ( s , other ) :
    """``dot''-product of two 3-vectors
    >>> v3    = ...
    >>> other = ...
    >>> print 'Dot is ' , p3.Dot ( other )
    """
    res  = s.X ( ) * other.X ( )
    res += s.Y ( ) * other.Y ( )
    res += s.Z ( ) * other.Z ( )
    return res