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
['dBdB dgfe', 'Bc dcBA B2 ed cBAB c2 dc']  
X:1
L:1/8
M:4/4
K:G
|: Bc | dBdB dgfe | dBdB dgfe | dBdB edcB | c2 A2 A2 Bc | dBdB dgfe | dBdB dgfe | decd BGAF | 
 G2 G2 G2 :: Bc | dcBA B2 ed | cBAB c2 dc | BGBG cBAG | FGAB A2 Bc | dcBA B2 ed | cBAB c2 dc | 
 BgGB AcFA | G2 G2 G2 :|  