Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
M:6/8
K:D
['DFA BAF AdF E2 D BAB dAF', 'faa bag fed']  
X:1
L:1/8
M:6/8
K:D
 DFA BAF | AdF E2 D | BAB dAF | EFE EDB, | DFA BAF | AdF E2 D | BAB dAF | EDD D3 :: faa bag | 
 fed Add | faa baf | fed e3 | faa bag | fed BAF | ABc dAF | EDD D3 :|  