Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
M:4/4
K:G
['G2 GB AGED', 'g2 eg']  
X:1
L:1/8
M:4/4
K:G
 G2 GB AGED | GGBd gedB | G2 GB AGED | cBAG E2 D2 :: g2 eg dgeg | g2 eg ea a2 |1 g2 eg dgeB | 
 cBAG EG D2 :|2 bgaf gfed | ^cdef (3gfe dB |]  