def initials(field, n):
    """predicate which returns first a tuple containing
    the first n chars of a field if and only if the
    field contains at least n characters, or an empty
    tuple otherwise.
    
    :param field: the string 
    :type n: int, default None
    
    usage:
    >>> initials("dedupe", 7)
    ()
    >>> initials("deduplication", 7)
    ('dedupli', )
    """
    return (field[:n], ) if len(field) > n-1 else ()