1. In a computer system for processing expressions entered by a user, the expressions comprising a plurality of tokens which evaluate to a result, said plurality of tokens including at least one operator token and at least one operand token, a method for assisting the user with correctly creating an expression, the method comprising: (a) storing a set of rules specifying valid relationships between adjacent tokens; (b) receiving user input for placing at least one token in said expression being created; (c) determining validity of said expression being created by applying at least one of said set of rules to adjacent ones of said at least one token; (d) if said expression being created is determined not to be valid in step (c), automatically modifying said expression so that validity of said expression being created is maintained; and (e) if modifying said expression in step (d) results in at least one addition or deletion of tokens from said expression being created, repeating steps (c) and (d) for all said at least one addition or deletion of tokens.