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
['A2 FG', 'fa a2']  
X:1
L:1/8
M:4/4
K:D
 Ad B/c/d A2 FG | FAdf geef | fdBd A/B/A FG | A2 de fd d2 | fdBd A2 FG | A/A/A df geef | 
 AdBd A2 FG | Adde fd d2 || fa a2 fa d2 | ef f/e/d ce A2 | fa a2 faad | ABde fd d2 | faad fada | 
 e2 ed ce A2 | B/c/d c2 d/e/f eg | fbag fd d2 ||  