Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
Q:3/8=50
M:3/8
K:G
['B2 G', 'd2 g e2 a', "b c'2"]  
X:1
L:1/8
Q:3/8=50
M:3/8
K:G
 d | d2 B | B2 G | AGA | B2 G | d2 B | B2 G | AGA | G2 :: d | d2 g | e2 a | fef | g2 b | d2 g | 
 e2 a | fef | g2 :: b | c'2 b | a2 g | f2 g | a2 b | c'2 a | b2 g | agf | g2 :|  