Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
M:4/4
K:D
['AF F2 Adfa g2 ag fddf', 'af f2 df f2 ge e2 ceAG']  
X:1
L:1/8
M:4/4
K:D
 AF F2 Adfa | g2 ag fddf | eA A2 d=cAF | G2 AF GFe=c | AF F2 Adfa | g2 ag fddf | ea a2 aged | 
 cAGE FD D2 :| af f2 df f2 | ge e2 ceAG | F2 AF GBed | ca^ga bagb | af f2 df f2 | ge e2 ceAG | 
 F2 AF GBed | cAGE FD D2 :|  