Develop a musical piece using the given theme and incorporating the letter-based structure provided.
X:1
L:1/4
Q:1/4=200
M:3/4
K:C
['f2 g a', '']  
X:1
L:1/4
Q:1/4=200
M:3/4
K:C
 C/ |[K:D] ue/d/"_see notes" d2 A | B g (f/g/) |"^=" e2 d | e/d/c/B/ A | d2 A | B g f | 
"^'" e d"^=" c | d3 ||"^=" f2 g | a e a |"^=" f2 g |"^E" a e a |"^=" f2 g |"^E" a (g/f/)(e/d/) | 
 (c/d/)"^=" (d3/2 c/4d/4) | e3 |]  