Using this Simulation you can get an complete idea about Decorder.

If you consider the process happening here is,

X=0 and Y=0 L1=On L2=Off L3=Off L4=Off

X=0 and Y=1 L1=Off L2=Off L3=On L4=Off

X=1 and Y=0 L1=Off L2=On L3=Off L4=Off

X=1 and Y=1 L1=Off L2=Off L3=Off L4=On