~(x | y) <==> ~x & ~y
~(x & y) <==> ~x | ~y
Then
~(~(x | y) & ~(y | z)) ==>
~((~x & ~y) & (~y & ~z)) ==>
~(~x & ~y) | ~(~y & ~z) ==>
~~x | ~~y | ~~y | ~~z ==>
x | y | y | z ==>
x | y | z
Applying the operations from the outside to the inside is faster:
~(~(x | y) & ~(y | z)) ==>
~~(x | y) | ~~(x | z) ==>
x | y | y | z