Consider a bidirectional shift register implemented with 4 D flip-flops. This circuit has the following I/O :

• R

  = direction control of the shift     (R = 1: shift right; R = 0: shift left))

• E_S

  = serial data input

• S

  _S

  = serial data output

• Q0 corresponds to the leftmost flip-flop
• Q3 corresponds to the rightmost flip-flop

What must be the logical function of the combinational circuit that generates the input D3 ?