Finish Scan documentation

1 files changed, 16 insertions, 4 deletions

diff --git a/doc/scan.md b/doc/scan.md
index ab4b6669..c4e3a3ad 100644
--- a/doc/scan.md
+++ b/doc/scan.md
@@ -50,7 +50,7 @@ BQN's Scan is ordered differently from Scan in APL. Both include one result for
 
 Scan also differs from Fold or Insert in that it never depends on `𝔽`'s identity element, because scanning over an empty array simply returns that array.
 
-## Examples
+## Lists
 
 The best-known use of Scan is the [prefix sum](https://en.wikipedia.org/wiki/Prefix_sum) of a list, in which each element of the result is the sum of that element and all the ones before it. With a [shift](shift.md) this can be modified to sum the previous elements only.
 
@@ -92,7 +92,7 @@ A more complicated boolean scan, which depends on the left-to-right ordering, is
 
         {¬<`'\'=𝕩}⊸/ "ab\\\rs\\\\"
 
-### Reverse scan
+## Reverse scan
 
 We've discussed how the scan moves forward along `𝕩`, so that each time `𝔽` takes an old result as `𝕨` and a new value as `𝕩`. This means that results correspond to prefixes and go left to right on each one. Since the most important scans have associative, commutative operands, the left-to-right ordering often doesn't make a difference. But sometimes a suffix rather than prefix scan is wanted. For these cases, Scan Under [Reverse](reverse.md) (`` `⌾⌽ ``) does the trick.
 
@@ -109,10 +109,22 @@ The new value is still the right argument to `𝔽`, even though with the revers
         {"("∾𝕨∾")𝔽"∾𝕩}˜`⌾⌽ "a"‿"b"‿"c"‿"d"
 
 
-### Higher ranks
+## Higher ranks
 
-Scan moves along the [leading axis](leading.md) of `𝕩`. Results are produced in index order.
+Scan moves along the [leading axis](leading.md) of `𝕩`: vertically, for a table. To apply a scan to later axes, use `˘` or `⎉`. Since a scan returns an array with the same shape as its argument, this can't cause an error from differing result cell shapes, unlike Fold or Insert.
 
         ⊢ a ← ¯2‿0.25‿'a'‿∞ ∾ 3‿4⥊¯1‿0‿1
 
         +` a
+
+If `𝕨` is given, it must have the same shape as a major cell of `𝕩` (this is why `𝕨` needs to be enclosed when `𝕩` is a list: in general it's an array). Then the first result cell is found by applying `𝔽` to elements of `𝕨` and `⊏𝕩`, and the computation continues as in the one-argument case for remaining cells.
+
+        3‿2‿1‿0 +` a
+
+Results are produced in index order. This means that instead of moving along each column in turn, a scan produces the first result cell one element at a time, then the next, and so on. Something like a breadth-first as opposed to depth-first ordering.
+
+## Definition
+
+Scan admits a simple recursive definition. `𝕩` is an array of rank one or more and `𝕨`, if given, is an atom or array with shape `1↓≢𝕩`. The result ``z←𝕨𝔽`𝕩`` is an array with the same shape as `𝕩`. If it has length at least one, `⊏z` is `⊏𝕩` if `𝕨` isn't given and `𝕨𝔽¨⊏𝕩` if it is. For `0≤i`, `(i+1)⊏z` is `(i⊏z)𝔽¨(i+1)⊏𝕩`.
+
+The ordering of `𝔽` application is the natural one for this definition: cells are computed in turn, and each instance of `𝔽¨` goes in index order.