Compose a piece of music that follows both the given musical theme and the letter-based structure provided.
X:1
L:1/8
Q:1/8=160
M:6/8
K:Bb
['a2 g', 'e2 f', 'd2 B c2', 'bab f3', 'b a2 g fga |1 b3 f3']  
X:1
L:1/8
Q:1/8=160
M:6/8
K:Bb
 B3 d2 B | d2 e f3 | def g2 b |1 a2 g f2 d :|2 a2 g f3 |: b3 f2 e | dcd B2 d |1 f3 agf | 
 g2 a bfd :|2 f3 a2 g | f3 f3 |: f2 g f2 d | e2 f g2 f | e2 f edc | d2 e f3 | d2 e d2 B | 
 c2 d e2 c | A2 c cdc | B3 B3 :: B3 f2 f | e2 g f3 |1 edc d2 B | c2 A B2 f :|2 def d2 B | 
 c2 A B3 |: bab f3 | ede f3 | gfg e2 d | c2 d e3 | bab f3 | def g2 b | a2 g fga |1 b3 f3 :|2 
 b3 b3 |]  