Rename and reformat amb/exists recipes

recipes/find-stride-across-lists.md

@@ -0,0 +1,42 @@
+# Find stride across lists
+
+## Problem
+
+I store a matrix as a list of rows, where each row is a list of column
+values for that row. I want to check if there is a column in the
+matrix that satisfies a comparator `p`.
+
+`p` can actually return any non-`#f` value and that value is returned
+as-is. And the lists don't actually have to represent a matrix: the
+rows don't all need to have the same number of columns.
+
+## Solution
+
+```scheme
+(define (find-stride p . a*)
+  (letrec
+    ((car-of
+       (lambda (a)
+         (map car a)))
+     (cdr-of
+       (lambda (a)
+         (map cdr a)))
+     (any-null
+       (lambda (a)
+         (memq '() a)))
+     (find*
+       (lambda (a*)
+         (and (not (any-null a*))
+              (or (apply p (car-of a*))
+                  (find* (cdr-of a*)))))))
+    (find* a*)))
+```
+
+Credit: [Nils M. Holm](https://t3x.org) (ref: [exists.scm](https://t3x.org/s9fes/exists.scm.html))
+
+## Usage
+
+```scheme
+(find-stride < '(9 1) '(8 2) '(7 3))
+;; ==> #t  ; because (< 1 2 3)
+```

recipes/amb.md → recipes/nondeterminism-using-amb.md

@@ -1,14 +1,20 @@
-# Create McCarthy's AMB operator.
+# Nondeterminism using amb
 
 ## Problem
 
-I want to implement amb operator, so I can use nondeterminism in my code.
+I want to use McCarthy's `amb` operator to express nondeterminism in
+my code.
 
 ## Solution
 
-This solution use code from:
-* [/use-list-as-stack/](Use list as Stack) - `push!` and `pop!`
-* [/find-if-element-in-list-exists](Find if element exists in list) - `exists?`
+This solution depends on the `syntax-rules` ellipsis extension that is
+present in R7RS and
+[SRFI 46](https://srfi.schemers.org/srfi-46/srfi-46.html).
+
+It also depends on two other recipes:
+
+* [Use list as stack](/use-list-as-stack/) - `push!` and `pop!`
+* [Find stride across lists](/find-stride-across-lists) - `find-stride`
 
 ```scheme
 (define *amb-stack* '())
@@ -54,11 +60,10 @@ This solution use code from:
             (amb))))))
 ```
 
-Code requires syntax-rules to support [SRFI-46](https://srfi.schemers.org/srfi-46/srfi-46.html) required by R7RS.
-
 Credit: [Jakub T. Jankiewicz](https://jcubic.pl/me)
 
-based on code by [Nils M Holm](https://t3x.org/) (ref: [amb.scm](http://t3x.org/s9fes/amb.scm.html))
+based on code by [Nils M Holm](https://t3x.org/)
+(ref: [amb.scm](https://t3x.org/s9fes/amb.scm.html))
 
 ## Usage
 

www-index.scm

@@ -1,8 +1,9 @@
 (("Pairs and lists"
   "create-k-combinations-from-list"
+  "find-depth-of-list"
   "find-index-of-element-in-list"
   "find-most-frequent-element-in-list"
-  "find-depth-of-list"
+  "find-stride-across-lists"
   "index-list"
   "remove-duplicates-from-list"
   "remove-element-from-list"
@@ -20,6 +21,9 @@
   "use-list-as-set"
   "use-list-as-queue")
 
+ ("Algorithms"
+  "nondeterminism-using-amb")
+
  ("Strings"
   "convert-any-value-to-string"
   "join-list-of-strings-with-delimiter"