Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
M:4/4
K:C
['G2 c2 cd e2 dc _B2 cA']  
X:1
L:1/8
M:4/4
K:C
 G2 | c2 cd e2 dc | _B2 cA F2 Ac | _B2 AF G2 c=B | c6 G2 | c2 cd e2 dc | _B2 cA F2 Ac | 
 _B2 AF G2 c=B | c6 ||  