Compose a piece of music that follows both the given musical theme and the letter-based structure provided.
X:1
L:1/8
M:4/4
K:D
['D2 AF', 'af f2']  
X:1
L:1/8
M:4/4
K:D
 D2 AF D2 AF | dBAF BE E2 | D2 AF D2 AF | dBAF DD D2 :: af f2 af f2 | dfaf ee e2 | af f2 afaf | 
 dfaf dd d2 | Af f2 afaf | dfaf ee e2 | afge fdec | dBAF DD D2 ||  