Develop a musical piece using the given theme and incorporating the letter-based structure provided.
X:1
L:1/4
Q:1/4=120
M:6/4
K:Bb
['d e f', 'B c']  
X:1
L:1/4
Q:1/4=120
M:6/4
K:Bb
 d e f f d B | d e f c2 B | d c d e f d/e/ | f c3/2 B/ B3 :| c3/2 d/ c c A F | F G A A B c | 
 B c d d e f | f e f d3/2 c/ B | c d e e3/2 f/ e | c d e c3/2 B/ A | B B c d d e | f c3/2 B/ B3 |]  