Write a Python function concat_corresponding( a, b ), which takes two strings, a and b and returns a string where every two-character slice is a concatenation of the corresponding elements at the same indices in a and b. You should assume that values in a shorter string are to be taken as empty strings. Examples: >>> print( concat_corresponding( "12345", "6789XYZ" ) ) 162738495XYZ >>> print( concat_corresponding( "12345", "678" ) ) 16273845 >>> print( concat_corresponding( "678", "12345" ) ) 61728345 ?