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', 'a3 f']  
X:1
L:1/8
M:4/4
K:D
 AF F2 AdFG | AFdF eFdF | AF F2 ABde | f2 af edBd :| defg a3 f | b3 f a3 f | defg a2 fa | 
 bfaf egfe | defg a3 f | b3 f a3 f | g3 e f3 e | defd efdB ||  