+/* --- @parse_char@ --- *
+ *
+ * Arguments: @union tvec_regval *rv@ = register value
+ * @const struct tvec_regdef *rd@ = register definition
+ * @struct tvec_state *tv@ = test-vector state
+ *
+ * Returns: Zero on success, %$-1$% on error.
+ *
+ * Use: Parse a register value from an input file.
+ *
+ * A character value can be given by symbolic name, with a
+ * leading `%|#|%'; or a character or `%|\|%'-escape sequence,
+ * optionally in single quotes.
+ *
+ * The following escape sequences and character names are
+ * recognized.
+ *
+ * * `%|#eof|%' is the special end-of-file marker.
+ *
+ * * `%|#nul|%' is the NUL character, sometimes used to
+ * terminate strings.
+ *
+ * * `%|bell|%', `%|bel|%', `%|ding|%', or `%|\a|%' is the BEL
+ * character used to ring the terminal bell (or do some other
+ * thing to attract the user's attention).
+ *
+ * * %|#backspace|%, %|#bs|%, or %|\b|% is the backspace
+ * character, used to move the cursor backwords by one cell.
+ *
+ * * %|#escape|% %|#esc|%, or%|\e|% is the escape character,
+ * used to introduce special terminal commands.
+ *
+ * * %|#formfeed|%, %|#ff|%, or %|\f|% is the formfeed
+ * character, used to separate pages of text.
+ *
+ * * %|#newline|%, %|#linefeed|%, %|#lf|%, %|#nl|%, or %|\n|% is
+ * the newline character, used to terminate lines of text or
+ * advance the cursor to the next line (perhaps without
+ * returning it to the start of the line).
+ *
+ * * %|#return|%, %|#carriage-return|%, %|#cr|%, or %|\r|% is
+ * the carriage-return character, used to return the cursor to
+ * the start of the line.
+ *
+ * * %|#tab|%, %|#horizontal-tab|%, %|#ht|%, or %|\t|% is the
+ * tab character, used to advance the cursor to the next tab
+ * stop on the current line.
+ *
+ * * %|#vertical-tab|%, %|#vt|%, %|\v|% is the vertical tab
+ * character.
+ *
+ * * %|#space|%, %|#spc|% is the space character.
+ *
+ * * %|#delete|%, %|#del|% is the delete character, used to
+ * erase the most recent character.
+ *
+ * * %|\'|% is the single-quote character.
+ *
+ * * %|\\|% is the backslash character.
+ *
+ * * %|\"|% is the double-quote character.
+ *
+ * * %|\NNN|% or %|\{NNN}|% is the character with code NNN in
+ * octal. The NNN may be up to three digits long.
+ *
+ * * %|\xNN|% or %|\x{NN}|% is the character with code NNN in
+ * hexadecimal.
+ */
+