Develop a musical piece using the given theme and incorporating the letter-based structure provided.
X:1
L:1/8
M:2/4
K:D
['F/G/ [FA][FA][FA]', 'f/g/ aaa']  
X:1
L:1/8
M:2/4
K:D
 F/G/ | [FA][FA][FA] F/G/ | [FA][FA][FA] d/c/ | B/A/G/F/ EA | FD D F/G/ | [FA][FA][FA] F/G/ | 
 [FA][FA][FA] d/f/ | edcB | A/^G/A/B/ A :: f/g/ | aaa f/g/ aaa a/f/ | eg/e/ ce/c/ | df/d/ A f/g/ | 
 aaa f/g/ | aaa g/a/ | bgec | d2 d :|  