
I would like you to implement a module named TopModule with the following
interface. All input and output ports are one bit unless otherwise
specified.

 - input  a
 - input  b
 - input  c
 - input  d
 - output q

The module should implement a combinational circuit. Read the simulation
waveforms to determine what the circuit does, then implement it.

  time  a  b  c  d  q
  0ns   0  0  0  0  0
  5ns   0  0  0  0  0
  10ns  0  0  0  0  0
  15ns  0  0  0  0  0
  20ns  0  0  0  1  0
  25ns  0  0  1  0  1
  30ns  0  0  1  1  1
  35ns  0  1  0  0  1
  40ns  0  1  0  1  1
  45ns  0  1  1  0  1
  50ns  0  1  1  1  1
  55ns  1  0  0  0  0
  60ns  1  0  0  1  0
  65ns  1  0  1  0  1
  70ns  1  0  1  1  1
  75ns  1  1  0  0  1
  80ns  1  1  0  1  1
  85ns  1  1  1  0  1
  90ns  1  1  1  1  1

