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authorNorbert Preining <norbert@preining.info>2019-09-02 13:46:59 +0900
committerNorbert Preining <norbert@preining.info>2019-09-02 13:46:59 +0900
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+[This was included as a self-decoding answer in the posting of Exercise
+#14; see exercise.014]
+
+Answer to Around the Bend #14:
+
+ Catcode Char Codes Catcode Char Codes
+ ------- ---------- ------- ----------
+ 1 0--255 10 1--255
+ 2 0--255
+ 3 0--255 11 0--255
+ 4 0--255 12 0--255
+ 13 0--255
+ 6 0--255
+ 7 0--255
+ 8 0--255
+
+Category 10 is the exceptional case. Catcode-10 characters with character
+code <> 32 can only be produced by \uppercase/\lowercase tricks (TeXbook,
+Appendix D). So the pair character 0, catcode 10 is not possible: \uppercase
+and \lowercase cannot produce a character 0 from a non-0 character.
+
+Active characters will test true for category 10 with \ifcat if they are
+\let equal to a space token. But if the ~ character (say) has been so
+defined, it will not match a space in the delimiter text of a macro with
+delimited arguments. And according to \tracingcommands the meaning of an
+active tilde that has been \let equal to a space is "blank space ",
+whereas the meaning of a category-10 tilde is "blank space ~".