Token delimiters.

Tokens should normally be delimited from surrounding text (such as variable names) by non-alphanumeric characters, otherwise they will not be recognised. The syntax of transformation functions ensures that this will normally be so, but, when this is not possible, enclosing angle brackets `$<$...$>$' may also be used as token delimiters. In this case the brackets will be replaced as if they were part of the token itself. By this means, values may be substituted to form part of a variable name if required.