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Merge pull request #112 from zcash/book-uncommitted-leaves 

[book] commitment-tree.md: Update Uncommitted^Orchard from 0 -> 2.
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book/src/design/commitment-tree.md
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@ -26,8 +26,8 @@ appended to the tree as a regular leaf.
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 $F_q$:
uncommitted leaf value. We use the value $2$ for both Pallas and Vesta, because $2^3 + 5$ is
not a square in either $F_p$ or $F_q$:
```python
sage: p = 0x40000000000000000000000000000000224698fc094cf91b992d30ed00000001
@ -36,12 +36,17 @@ 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()
sage: Mod(13, p).is_square()
False
sage: Mod(5, q).is_square()
sage: Mod(13, q).is_square()
False
```
> Note: There are also no Pallas points with $x$-coordinate $0$, but we map the identity to
> $(0, 0)$ within the circuit. Although $\mathsf{SinsemillaCommit}$ cannot return the identity
> (the incomplete addition would return $\perp$ instead), it would arguably be confusing to
> rely on that.
## Considered alternatives
We considered splitting the commitment tree into several sub-trees: