def parse_field(source, loc, tokens):
    """
    Returns the tokens of a field as key-value pair.
    """
    name = tokens[0].lower()
    value = normalize_value(tokens[2])
    if name == 'author' and ' and ' in value:
        value = [field.strip() for field in value.split(' and ')]
    return (name, value)
Returns the tokens of a field as key-value pair.