def tokenize(line):
    """Convert a string into a list of tokens.

    >>> tokenize('2.3')
    [2.3]
    >>> tokenize('(2 3)')
    ['(', 2, 3, ')']
    >>> tokenize('<2 3)')
    ['<', 2, 3, ')']
    >>> tokenize('<[2{12.5 6.0}](3 -4 5)>')
    ['<', '[', 2, '{', 12.5, 6.0, '}', ']', '(', 3, -4, 5, ')', '>']

    >>> tokenize('2.3.4')
    Traceback (most recent call last):
        ...
    ValueError: invalid token 2.3.4

    >>> tokenize('?')
    Traceback (most recent call last):
        ...
    ValueError: invalid token ?

    >>> tokenize('hello')
    Traceback (most recent call last):
        ...
    ValueError: invalid token hello

    >>> tokenize('<(GO BEARS)>')
    Traceback (most recent call last):
        ...
    ValueError: invalid token GO
    """
    # Surround all brackets by spaces so that they are separated by split.
    for b in all_brackets:
        line = line.replace(b, ' ' + b + ' ')

    # Convert numerals to numbers and raise ValueErrors for invalid tokens.
    tokens = []
    for t in line.split():
        if t in all_brackets:
            tokens.append(t)
        elif coerce_to_number(t) != None:
            tokens.append(coerce_to_number(t))
        else:
            raise ValueError('invalid token %s' % (t) )
    return tokens