+A symbol or label is an identifier that starts with a letter and is followed
+by letters and digits. Depending on some features enabled (see
+<tt><ref id="at_in_identifiers" name="at_in_identifiers"></tt>,
+<tt><ref id="dollar_in_identifiers" name="dollar_in_identifiers"></tt> and
+<tt><ref id="leading_dot_in_identifiers" name="leading_dot_in_identifiers"></tt>)
+other characters may be present. Use of identifiers consisting of a single
+character will not work in all cases, because some of these identifiers are
+reserved keywords (for example "A" is not a valid identifier for a label,
+because it is the keyword for the accumulator).
+