Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
Q:1/8=300
M:6/8
K:none
['', 'BGG DGG BGB dcB', 'g/a/ bgg dgg bgb bag aff dff']  
X:1
L:1/8
Q:1/8=300
M:6/8
K:none
[K:G]|: d/c/ | BGG DGG | BGB dcB | cAA DAA | cAc edc | BGG DGG | BGB dcB | cBc Adc | BGG G2 :: 
 g/a/ | bgg dgg | bgb bag | aff dff | fdf agf | egg dgg | cgg Bgg | dcB Adc | BGG G2 :|  