Docs: Fix word_logic.rst

It somehow got lost in the rebase.

docs/source/cell/word_logic.rst

@@ -0,0 +1,46 @@
+Arbitrary logic functions
+-------------------------
+
+The `$lut` cell type implements a single-output LUT (lookup table). It
+implements an arbitrary logic function with its ``\LUT`` parameter to map input
+port ``\A`` to values of ``\Y`` output port values. In psuedocode: ``Y =
+\LUT[A]``. ``\A`` has width set by parameter ``\WIDTH`` and ``\Y`` has a width
+of 1. Every logic function with a single bit output has a unique `$lut`
+representation.
+
+The `$sop` cell type implements a sum-of-products expression, also known as
+disjunctive normal form (DNF). It implements an arbitrary logic function. Its
+structure mimics a programmable logic array (PLA). Output port ``\Y`` is the sum
+of products of the bits of the input port ``\A`` as defined by parameter
+``\TABLE``. ``\A`` is ``\WIDTH`` bits wide. The number of products in the sum is
+set by parameter ``\DEPTH``, and each product has two bits for each input bit -
+for the presence of the unnegated and negated version of said input bit in the
+product. Therefore the ``\TABLE`` parameter holds ``2 * \WIDTH * \DEPTH`` bits.
+
+For example:
+
+Let ``\WIDTH`` be 3. We would like to represent ``\Y =~\A[0] + \A[1]~\A[2]``.
+There are 2 products to be summed, so ``\DEPTH`` shall be 2.
+
+.. code-block::
+
+    ~A[2]-----+
+     A[2]----+|
+    ~A[1]---+||
+     A[1]--+|||
+    ~A[0]-+||||
+     A[0]+||||| 
+         |||||| product formula
+         010000 ~\A[0]
+         001001 \A[1]~\A[2]
+
+So the value of ``\TABLE`` will become ``010000001001``.
+
+Any logic function with a single bit output can be represented with ``$sop`` but
+may have variously minimized or ordered summands represented in the ``\TABLE``
+values.
+
+.. autocellgroup:: logic
+   :members:
+   :source:
+   :linenos: