def TR4(rv):
    """Identify values of special angles.

        a=  0   pi/6        pi/4        pi/3        pi/2
    ----------------------------------------------------
    cos(a)  0   1/2         sqrt(2)/2   sqrt(3)/2   1
    sin(a)  1   sqrt(3)/2   sqrt(2)/2   1/2         0
    tan(a)  0   sqt(3)/3    1           sqrt(3)     --

    Examples
    ========

    >>> for s in (0, pi/6, pi/4, pi/3, pi/2):
    ...     print(f'{cos(s)} {sin(s)} {tan(s)} {cot(s)}')
    ...
    1 0 0 zoo
    sqrt(3)/2 1/2 sqrt(3)/3 sqrt(3)
    sqrt(2)/2 sqrt(2)/2 1 1
    1/2 sqrt(3)/2 sqrt(3) sqrt(3)/3
    0 1 zoo 0

    """
    return rv
