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
['dAFA DFAc dcec dAA=c BAGF GABc |1', 'fdad fdad']  
X:1
L:1/8
M:4/4
K:D
 dAFA DFAc | dcec dAA=c | BAGF GABc |1 (3ddd ed cABc :|2 (3ddd ec d2 || fg | fdad fdad | 
 fdad fddf | e=cgc ecgc | e=cgc ecAc | fdad fdad | fdad fddf | g2 gf gbag | fdec d2 Ac |]  