Russell identifiers must consist of one of the following:
 
        1) an alphabetic character followed by 0 or more alphabetic
           characters and digits.  "_" is an alphabetic character.

        2) 0 or more characters (including \n, \t, \r, \') enclosed in
           single quotes.

        3) A sequence of one or more operator characters.  These are
                ! % & * + - . / : < = > ? @ \ ^ ` | ~

Any identifier occurring as an expression may optionally be followed
by

        << signature >>

to explicitly indicate which, of a number of overloaded instances
of the identifier, was intended.