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
['FG ABAF Adcd', 'd2 cd ea^ga e=gfe d2 fd adfd']  
X:1
L:1/8
M:4/4
K:D
|: FG | ABAF Adcd | B2 BA BdcB | ABde fedf | e2 E2 E2 FG | ABAF Adcd | Bggf gBdg | fAdf eAce | 
 d2 d2 d2 :: cd | ea^ga e=gfe | d2 fd adfd | gfeg f2 df | efed B2 dB | ABAF Adcd | Bggf gBdg | 
 fAdf eAce | d2 d2 d2 :|  