D I Y A
D, DI, DY, DA, DIY, DIA, DYA, DIYA
I, IY, IA, IYA
Y, YA
A
null ( 0 length)
HERE we can see total 16 subsequences are possible.
there is a difference between sub-string and subsequence, sub-string should be continuous but subsequence may or may not be.
so, string length = n
total subsequences possible = 2$^{n}$