# shift_lowerdef shift_lower(s1, s2):

"""shift_lower. This function must return a string containing the lowercase letters of s1 followed by the lowercase letters of s2 followed by the uppercase letters of s1 followed by the uppercase letters of s2. Do not otherwise change the order of the letters (see the test cases below for examples).

Parameters:

s1 (str): A string of characters, potentially including non-letters that shouldbe ignored

s2 (str): A string of characters, potentially including non-letters that shouldbe ignored

Returns:

shifted (str): A string of letters or "" as per the description above.

"""

return