+
+If an unknown mark contains a quote character or a comment marker, and
+that token is not being ignored, then we terminate the unknown mark
+before that quote or comment. This ensures that an unknown mark
+immediately before a string is handled correctly.
+
+If the first character of a comment marker (i.e. '/') is a known mark,
+the above rules would suggest that the start of a comment would be
+parsed as that mark, which is not what is wanted. So the introductory
+sequences for a comment ("//" and "/*") are treated as
+partially-known. They prevent the leading "/" from being a mark by
+itself, but do not actually constitute a stand-alone mark.
+
+If `TK_mark` is ignored, then unknown marks are returned as errors.