Compose a piece of music that follows both the given musical theme and the letter-based structure provided.
X:1
L:1/8
M:3/4
K:D
['f/g/af geg']  
X:1
L:1/8
M:3/4
K:D
 A/G/ | FDD D2 E | F/G/AF G/A/BA | BEF G2 A/G/ | FED d2 e | fed edc | dBe cAc | dDD D2 | f/g/ | 
 afa dfa | bgb efg | faf ged | cAA A2 g | f/g/af geg | f/g/af geg | fed cBc | dDD D3 |]  