BIP-0158: switch to prev output scripts, skip all OP_RETURN

Author: Roasbeef

bip-0158.mediawiki

@@ -273,10 +273,22 @@ This BIP defines one initial filter type:
 ==== Contents ====
 
 The basic filter is designed to contain everything that a light client needs to
-sync a regular Bitcoin wallet. A basic filter MUST contain exactly the following
-items for each transaction in a block:
-* The outpoint of each input, except for the coinbase transaction
-* The scriptPubKey of each output
+sync a regular Bitcoin wallet. A basic filter MUST contain exactly the
+following items for each transaction in a block:
+* The previous output script (the script being spent) for each input, except
+  for the coinbase transaction.
+* The scriptPubKey of each output, aside from all <code>OP_RETURN</code> output
+  scripts.
+
+Any "nil" items MUST NOT be included into the final set of filter elements. 
+
+We exclude all <code>OP_RETURN</code> outputs in order to allow filters to
+easily be committed to in the future via a soft-fork. A likely area for future
+commitments is an additional <code>OP_RETURN</code> output in the coinbase
+transaction similar to the current witness commitment
+<ref>https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki</rev>. By
+excluding all <code>OP_RETURN</code> outputs we avoid a circular dependency
+between the commitment, and the item being committed to.
 
 ==== Construction ====