Develop a musical piece using the given theme and incorporating the letter-based structure provided.
X:1
L:1/8
M:2/2
K:Bb
['BA B2 B,2 B,2 FE DF B2', 'B2 FE DFBF DFBF G2 E2 E2']  
X:1
L:1/8
M:2/2
K:Bb
 BA | B2 B,2 B,2 FE | DF B2 B2 dB | c2 C2 C2 cB | Ac f2 f2 ga | bagf gfed | edcB BAGF | GBAc Bdce | 
 d2 B2 B2 :: FE | DFBF DFBF | G2 E2 E2 GF | =EGcG EGcG | A2 F2 F2 ed | e2 g2 gfed | edcB BAGF | 
 GBAc Bdce | d2 B2 B2 :|  