Develop a musical piece using the given theme and incorporating the letter-based structure provided.
X:1
L:1/16
Q:1/4=100
M:2/4
K:A
['c2c2 dcBA e2c2 a2gf', 'defd cdec B2B2']  
X:1
L:1/16
Q:1/4=100
M:2/4
K:A
"^moderate" AB | c2c2 dcBA | e2c2 a2gf | e2A2 dcBA | G2B2 B2AB |"^5" c2c2 dcBA | e2c2 a2gf | 
 ecBA GBcd | c2A2 A2 || ab |"^9" c2d2 c2a2 | e2c2 dcBA | c2d2 c2a2 | e2c2 e2dc |"^13" defd cdec | 
 B2B2 B2Bc | defd cdec | B2B2 B2AB |"^17" c2A2 e2A2 | fgaf e2AB | c2A2 e2A2 | fgaf e2dc | 
"^21" defd c2a2 | B2B2 B2Bc | defd c2a2 | c2A2 A4 |]  