<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def category_label(arg, labels, nulls=None):
 """ Format a known number of categories as strings Parameters labels : list of string nulls : string, optional How to label any null values among the categories Returns ------- string_categories : string value expression """

    op = CategoryLabel(arg, labels, nulls)
    return op.to_expr()