code = '1'
['expression', ['number', ['digit', '1']]]

code = '(1+2)'
['expression',
 '(',
 ['expression', ['number', ['digit', '1']]],
 ['operator', '+'],
 ['expression', ['number', ['digit', '2']]],
 ')']

code = '((12+34)*(56-78))'
['expression',
 '(',
 ['expression',
  '(',
  ['expression', ['number', ['digit', '1'], ['digit', '2']]],
  ['operator', '+'],
  ['expression', ['number', ['digit', '3'], ['digit', '4']]],
  ')'],
 ['operator', '*'],
 ['expression',
  '(',
  ['expression', ['number', ['digit', '5'], ['digit', '6']]],
  ['operator', '-'],
  ['expression', ['number', ['digit', '7'], ['digit', '8']]],
  ')'],
 ')']

code = '(1+'
Traceback (most recent call last):
  File "/Users/mahoney/Desktop/recursive_descent.py", line 185, in <module>
    pretty_print(expression(LexerQueue(code)))
  File "/Users/mahoney/Desktop/recursive_descent.py", line 148, in expression
    expr2 = expression(lq)
  File "/Users/mahoney/Desktop/recursive_descent.py", line 154, in expression
    return ["expression", number(lq)]
  File "/Users/mahoney/Desktop/recursive_descent.py", line 163, in number
    result = ['number', digit(lq)]
  File "/Users/mahoney/Desktop/recursive_descent.py", line 170, in digit
    assert value in digit_tokens, \
AssertionError: OOPS - didn't find expected digit