example/is-pangram now has commentary

FossilOrigin-Name: f4990f4a3aaa62bcd324c139b92ff910744679b101b9abd17a8fb06e7b536e98

example/is-pangram.forth

@@ -1,14 +1,52 @@
+A pangram is a scentence that uses all the letters in the alphabet.
+
+This is one way to determine if a scentence is a pangram using RETRO.
+
+First, define a string containing the alphabet:
+
 ~~~
-{{
+'abcdefghijklmnopqrstuvwxyz 'FULL s:const
-  'abcdefghijklmnopqrstuvwxyz 'FULL s:const
-  '__________________________ 'TEST s:const
----reveal---
-  :s:pangram? (s-f)
-    '__________________________ &TEST #26 copy
-    s:to-lower [ c:letter? ] s:filter [ dup $a - &TEST + store ] s:for-each
-    &TEST &FULL s:eq? ;
-}}
 ~~~
 
+Then a blank string of the same length for the test data:
+
+~~~
+'__________________________ 'TEST s:const
+~~~
+
+Now a word to do the actual test.
+
+~~~
+:s:pangram? (s-f)
+  '__________________________ &TEST #26 copy
+  s:to-lower [ c:letter? ] s:filter
+  [ dup $a - &TEST + store ] s:for-each
+  &TEST &FULL s:eq? ;
+~~~
+
+Breaking this down, the first line:
+
+  '__________________________ &TEST #26 copy
+
+Copies a blank string over the TEST string. Then:
+
+  s:to-lower [ c:letter? ] s:filter
+
+Converts the string to lowercase and strips out anything that's not a
+letter. Then a quick iteration over each character:
+
+  [ dup $a - &TEST + store ] s:for-each
+
+Reduces the letter to an index into the TEST string and stores the
+letter at the appropriate spot. And finally:
+
+  &TEST &FULL s:eq? ;
+
+Just compares the TEST and FULL strings to get the result.
+
+Here's a couple of test cases:
+
+~~~
 'Hello_world! s:pangram?
 'The_quick_brown_fox_jumped_over_the_lazy_dogs. s:pangram?
+~~~