Develop a musical composition using both the specified melodic theme and the alphabetical pattern naming.
X:1
L:1/8
M:6/8
K:Bb
['fBB (BAB)', 'c dcB', 'gdB gdB']  
X:1
L:1/8
M:6/8
K:Bb
 g | fBB (BAB) | fBB gBB | fBB (BAB) | cAF FAc | fBB (BAB) | fBB gBB | bag fed |1 cAF F2 :|2 
 cBA GFD |: B3 BFD | BFD B2 c | dcB BAB | cAF FGA | BFD BFD | BFD B2 c | dcB ABc |1 B3 Bdc :|2 
 B3 B3 ||[K:G] | gdB gdB | gdB d2 a | _bag gfg | afd def | gdB gdB | gdB g2 a | _bag fga |1 
 g3 G2 a :|2 g3{cd} =e2 |]  