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
['(.vG2 .G) (B/c/)dB GBG (B2{cB} A) GBG', 'def gfe', 'ABc BGG ADD', 'gdB GAB']  
X:1
L:1/8
M:6/8
K:G
|: (.vG2 .G) (B/c/)dB | GBG (B2{cB} A) | GBG GdB | A=FA (c2 A) | G3 GdB | GAB gdB | cac BgB | 
 A=FA (c2 A) :: vg3 def | gfe dBG | g3 de=f | A=FA (c2 A) | g3 def | gfe dcB | aca BgB | 
 A=FA (c2 A) :: vBGG ADD | BGG ABc | BGG ADD | A=FA ABc | BGG ADD | GAB gdB | cac BgB | 
 A=FA (c2 A) :: vgdB gdB | GAB (d2 B) | gdB gdB | A=FA (c2 A) | gdB gdB | GAB gdB | cac BgB | 
 A=FA (c2 A) :|  