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
['Af f/e/f']  
X:1
L:1/8
M:2/4
K:D
"D" f/g/f/e/ d>B | AD FA |"G" GE e>d |"A" cB BA |"D" f/g/f/e/ d>B | AD FA |"G" GE"Em" e>f | 
"D" ed d3/2 :| B/ |"D" Af f/e/f |"G" Ag g/f/g |"D" Af f/e/f |"Em" e/f/e/d/"A" BA |"D" Af f/e/f | 
"G" Ag g>a |"A" ba gc | ed"D" d3/2 :|  