book: Note that we use 0 for uncommitted leaves in the commitment tree

book/src/design/commitment-tree.md

@@ -29,3 +29,33 @@ clients can assert they are valid independently of the full block.
 TODO: Sean is pretty sure we can just improve the Incremental Merkle Tree implementation
 to work around this, without domain-separating the tree. If we can do that instead, it may
 be simpler.
+
+## Uncommitted leaves
+
+The fixed-depth incremental Merkle trees that we use (in Sprout and Sapling, and again in
+Orchard) require specifying an "empty" or "uncommitted" leaf - a value that will never be
+appended to the tree as a regular leaf.
+
+- For Sprout (and trees composed of the outputs of bit-twiddling hash functions), we use
+  the all-zeroes array; the probability of a real note having a colliding note commitment
+  is cryptographically negligible.
+- For Sapling (where leaves are u-coordinates of Jubjub points), we use the value $1$
+  (which is not the u-coordinate of any Jubjub point).
+
+Orchard note commitments are the x-coordinates of Pallas points; thus we take the same
+approach as Sapling, using a value that is not the x-coordinate of any Pallas point as the
+uncommitted leaf value. It happens that $0$ is the smallest such value for both Pallas and
+Vesta, because $0^3 + 5$ is not a square in either $F_p$ or $F_q$:
+
+```python
+sage: p = 0x40000000000000000000000000000000224698fc094cf91b992d30ed00000001
+sage: q = 0x40000000000000000000000000000000224698fc0994a8dd8c46eb2100000001
+sage: EllipticCurve(GF(p), [0, 5]).count_points() == q
+True
+sage: EllipticCurve(GF(q), [0, 5]).count_points() == p
+True
+sage: Mod(5, p).is_square()
+False
+sage: Mod(5, q).is_square()
+False
+```