<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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)