Develop a musical piece using the given theme and incorporating the letter-based structure provided.
X:1
L:1/8
M:4/4
K:D
['e3 f']  
X:1
L:1/8
M:4/4
K:D
|: A2 | d2 cd ed cd | e3 f e2 A2 | f3 a g2 f2 | e4- e3 f | g2 e2 dc BA | f2 d2 A2 ^GA | 
 B2 g2 f2 e2 | d6 :|  