The function f(x) takes a word as input and applies a rule to it. The rule is that if the input word is a proper noun (i.e., the name of a specific person, place, or thing), the function returns the most specific label that describes the input word. For example, the word "paris" is a proper noun, so the function returns the label "city". Similarly, the word "obama" is a proper noun, so the function returns the label "president".

