Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/4
M:3/1
K:G
['A4 G8']  
X:1
L:1/4
M:3/1
K:G
 D4 | G6 G2 G4 | A8 B4 | c8 c4 | A8 A4 | B4 B4 A4 | G8 G4 | B8 c4 | d8 d4 | d6 c2 B4 | A8 B4 | 
 c8 B4 | G8 B4 | c4 c4 A4 | G8 B4 | c8 A4 | G8 B4 | c6 B2 A4 | G8 G4 | G4 G4 A4 | B8 c4 | d8 B4 | 
 c4 A8 | G8 |]  