The function f(x) takes a string input and performs a simple substitution of certain words in the input string with their corresponding uppercase or lowercase version. The function appears to follow a specific pattern of substitution, where words that start with a capital letter are replaced with their corresponding lowercase version, while words that start with a lowercase letter are replaced with their corresponding uppercase version. For example, the word "hello" is replaced with "ehAllo", while the word "python" is replaced with "typAhon".

