From eac71d728f7a83c52deddcc766ae1ad3f8e3ef23 Mon Sep 17 00:00:00 2001
From: Marshall Lochbaum <mwlochbaum@gmail.com>
Date: Sun, 14 Feb 2021 21:18:37 -0500
Subject: More detailed/correct description of Group

---
 spec/primitive.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

(limited to 'spec')

diff --git a/spec/primitive.md b/spec/primitive.md
index ba7a7805..fca283d5 100644
--- a/spec/primitive.md
+++ b/spec/primitive.md
@@ -161,7 +161,7 @@ For **Select** (`⊏`), `𝕨` is an array of natural numbers, or a list of such
 
 **First Cell** (`⊏`) selects the initial major cell of `𝕩`, giving an error if `𝕩` has rank 0 or length 0.
 
-**Group** (`⊔`) performs an opposite operation to Select, so that `𝕨` specifies not the argument index that result values come from, but the result index that argument values go to. The result rank is `≠𝕨`, and each result element has rank `1+(=𝕩)-+´=¨𝕨`. The result element contains the minimal list of cells so that each cell of `𝕩` appears in the corresponding element of `𝕨⊏𝕨⊔𝕩`, in index order. **Group Indices** treats its argument `𝕩` as a left argument for Group and uses a right argument made up of indices, with the definition `⊔⟜(↕≠⚇1)`.
+**Group** (`⊔`) performs an opposite operation to Select, so that `𝕨` specifies not the argument index that result values come from, but the result index that argument values go to. The general case is that `𝕨` is a list of arrays of numbers; if it has depth less than 2 it's converted to this form by first enclosing it if it's an atom, then placing it in a length-1 list. After this transformation, the result rank is `≠𝕨`, and each result element has rank `(≠𝕨)+(=𝕩)-+´=¨𝕨`, with the initial `≠𝕨` axes corresponding to elements of `𝕨` and the remainder to trailing axes of `𝕩`. Each atom in `𝕨` can be either a natural number or `¯1` (which indicates the corresponding position in `𝕩` will be omitted). If `¯1` doesn't appear, the result has the property that each cell of `𝕩` appears in the corresponding element of `𝕨⊏𝕨⊔𝕩`. More concretely, the length of the result along axis `a` is the maximum value in `a⊑𝕨` plus one, or zero if `a⊑𝕨` is empty. Axis `a` corresponds to `=a⊑𝕨` axes in `𝕩`, and an element of the result at position `i` along this axis contains all positions in `𝕩` where `i=a⊑𝕨`. There may be multiple such positions, and they're arranged along axis `a` of that result element according to their index order in `𝕩`. **Group Indices** treats its argument `𝕩` as a left argument for Group and uses a right argument made up of indices, with the definition `⊔⟜(↕≠⚇1)`.
 
 **Range** (`↕`) is extended to apply to a list of natural numbers, in addition to the provided case of a single natural number (an enclosed natural number `𝕩` should still result in an error). For a list `𝕩`, the result is an array of shape `𝕩` in which the value at a given index is that index, as a list of natural numbers. That is, `i≡i⊑↕𝕩` for any list of natural numbers `i` with `∧´i<𝕩`.
 
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