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A question about category name in ELisp

From: Xue Fuqiao
Subject: A question about category name in ELisp
Date: Sat, 5 Oct 2013 18:43:27 +0800


I have a question about category and category table.  In (info
"(elisp) Categories"):

    Each category has a name, which is an ASCII printing character in
  the range ‘ ’ to ‘~’.  You specify the name of a category when you
  define it with ‘define-category’.
    The category table is actually a char-table (*note Char-Tables::).
  The element of the category table at index C is a "category set"—a
  bool-vector—that indicates which categories character C belongs to.
  In this category set, if the element at index CAT is ‘t’, that means
  category CAT is a member of the set, and that character C belongs to
  category CAT.

>From the second paragraph, my impression about a category table is
something like this:

|     | a   | -   | .   | "   |
| bv1 | bv2 | bv3 | bv4 | bv5 |

(The first row contains the indices of the char-table (i.e., the
"index C" above), and the second row contains the elements at index
C1, C2, ... (i.e., it's a category set, which is a bool-vector.))

As with all arrays, bool-vector indices start from 0, so CAT should be
an integer.  But according to my understanding, CAT is a category
name, which should be an ASCII printing character (instead of just an
integer, from the first paragraph).  Why?  Am I missing something?

Best regards, Xue Fuqiao.

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