This is a temporary mini-program to demo how to get the complete sets of given elements, and the element used in the program is the letter,
for examples, if given three element {"a", "b", "c"}, then this program will output as following:
 { a }
 { b }
 { c }
 { a b }
 { b c }
 { a c }
 { a b c}
 
 Which denotes the all subsets of any combination of given elements (a, b, c),  the function is implemented as a recursive function.
 Thanks.