Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
M:6/8
K:G
['G2 A BAG', 'dBd gdB']  
X:1
L:1/8
M:6/8
K:G
 G2 A BAG | A2 B cBA | dBd gdB | AdB BAF | G2 A BAG | dcB ABd | gaf gdB | A2 B cBA :| dBd gdB | 
 ded dBc | dBd gdB | A2 B cBA | dBd gdB | ded def | gfa gdB | A2 B cBA | dBd gdB | ded dBc | 
 dBd gdB | A2 B cBA | dBd ece | fdf g2 a | bge dBG | A2 B cBA ||  