Add: Rationale section

bip-PC.md

@@ -32,12 +32,6 @@ Using in sequence `OP_CHECKTEMPLATEVERIFY`, `OP_PAIRCOMMIT`, `OP_INTERNALKEY`
 and `OP_CHECKSIGFROMSTACK` we can construct a rebindable channel that is also
 optimal.
 
-If `OP_CAT` was available, it could be used to combine multiple stack elements,
-that get verified with `OP_CHECKSIGFROMSTACK` as a valid state update.
-
-`OP_PAIRCOMMIT` solves this specific problem without introducing a wide range
-of potentially controversial new behaviors, such as novel 2-way peg mechanisms.
-
 The number of SHA256 iterations is minimized in the primary use case we
 can optimize for, which is LN-Symmetry. Since the Tag can be pre-computed as
 mid-state, it would only take 1 or 2 hash cycles in validation for the
@@ -132,6 +126,37 @@ A reference implementation is provided here:
 
 https://github.com/lnhance/bitcoin/pull/6/files
 
+## Rationale
+
+If `OP_CAT` was available, it could be used to combine multiple stack elements,
+that get verified with `OP_CHECKSIGFROMSTACK` as a valid state update.
+
+`OP_PAIRCOMMIT` solves this specific problem without introducing a wide range
+of potentially controversial new behaviors, such as novel 2-way peg mechanisms.
+
+### Behaviours LNhance tries to avoid introducing
+
+The following behaviors are out of scope for LNhance and should not be enabled
+as a side effect without explicit consensus:
+
+* Fine grained introspection
+* State carrying covenants
+* Bigint operations
+* New arithmetic capabilities using lookup tables
+
+### Alternative approaches
+
+The following list of alternative approaches were discussed and rejected for
+various reasons, either for expanding the scope or for unnecessary complexity:
+
+* OP_CAT
+* SHA256 streaming opcodes
+* Merkle operation opcodes
+* 'Kitty' CAT: result or inputs arbitrarily limited in size
+* OP_CHECKTEMPLATEVERIFY committing to the taproot annex in tapscript
+* OP_CHECKSIGFROMSTACK on n elements as message
+* OP_VECTORCOMMIT: generalized form for n > 2 elements
+
 ## Backward Compatibility
 
 By constraining the behavior of OP_SUCCESS opcodes, deployment of the BIP