Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/4
M:none
K:D
['G G']  
X:1
L:1/4
M:none
K:D
 D | =F F G G | A A B B | =c c B B A | G G | A A A A _B =c | _B A G | G G A =F | =F E D A | 
 G G =F E G | =F F D =C C | G G A A | =c A A A | G G G =F D | =F E D |]  