#lang racket
; Grammar:
; Terminals = {+, -, *, /, ^, (, )} U letters U digits
; Non-Terminals = {quantity, additive, product, multiplicative, power, primary,
;                  number, unit, exponent}
; Start symbol = quantity
; Rules:
;     quantity        -> product additive
;
;     additive        -> + product additive 
;                      | - product additive 
;                      | empty
;
;     product         -> power multiplicative
;
;     multiplicative  -> * power multiplicative 
;                      | / power multiplicative 
;                      | space power multiplicative 
;                      | empty
;
;     power           -> primary ^ exponent 
;                      | primary
;
;     primary         -> number 
;                      | unit 
;                      | ( quantity )
;
;     exponent        -> number
;
;     number          -> digit number 
;                      | digit
;
;     unit            -> letter unit
;                      | letter
;
; Eventually we will want to differentiate between exponent and number
; as exponent will always be an integers, whereas number can have a fraction.
;
; empty designates the empty string.
; space designates a space character.
; Whitespace is omitted for clarity, but it can optionally precede any item
; on the right-hand side, except for digit and letter.