Utilizing the provided musical theme and letter-based structure, compose a piece of music.
X:1
L:1/8
M:4/4
K:Bb
['B2 dB B2', 'B3 f bB B,2 bB(ba) gB G,2']  
X:1
L:1/8
M:4/4
K:Bb
 F | B2 dB B2 fB | B2 bg fedc | B2 dB B2 gB | AcFA B3 :| f | bB B,2 bB(ba) | gB G,2 gG(gf) | 
 ecdB cABb | agf=e f2 fa | bBAB GBEB | gc=Bc AcFA | BFdB fdbg | afga b2 B ||  