25. A development system for creating an expression from data entered by a user, said expression 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, said system comprising: a computer having a processor and a memory; means for storing in said memory information specifying valid relationships between tokens; input means for receiving user input for placing at least one token in said expression being created; means for determining validity of said expression being created by applying said rules to adjacent ones of said at least one token; means for automatically modifying said expression so that validity of said expression being created is maintained; and means for placing at least one proxy token for a token whose absence from the expression would render the expression invalid or incomplete.
25. A development system for creating an expression from data entered by a user, said expression 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, said system comprising: a computer having a processor and a memory; means for storing in said memory information specifying valid relationships between tokens; input means for receiving user input for placing at least one token in said expression being created; means for determining validity of said expression being created by applying said rules to adjacent ones of said at least one token; means for automatically modifying said expression so that validity of said expression being created is maintained; and means for placing at least one proxy token for a token whose absence from the expression would render the expression invalid or incomplete. 26. The system of claim 25, further comprising: means for indicating to the user on a display screen what input is needed to render the expression valid and complete.