Develop a musical piece using the given theme and incorporating the letter-based structure provided.
X:1
L:1/8
M:4/4
K:Amix
['A2 eg', 'a2 eg']  
X:1
L:1/8
M:4/4
K:Amix
 A2 eg fdeA | (3Bcd ef gfgA | A2 eg fdfa | gedg eA A2 :| a2 eg fdgf | eA (3cBA (3Bcd eg | 
 a2 eg fdfa | gedg eA A2 :|  