Compose a piece of music that follows both the given musical theme and the letter-based structure provided.
X:1
L:1/4
M:6/4
K:G
['A G']  
X:1
L:1/4
M:6/4
K:G
 D | G F G d c d | _B2 A G2 G | A G A d c d | A3 z2 A | _B A G G A B | c3 d2 c | _B A G F/E/ D d | 
 _B c B A G F | G3 z2 |]  