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
['B2 BA BAFB AFEF DEFA', 'd2 fd']  
X:1
L:1/8
M:4/4
K:D
 z | B2 BA BAFB | AFEF DEFA | B4 c3 d | dcBc BAFE | B2 BA BAFB | AFEF DEFA | dfec dBAG | FAEA D3 :| 
 A | d2 fd ed f2 | fgaf gfef | d2 fd gd f2 | BAGB AFFA | d2 fd ed f2 | fgaf gfef | fdec dBAG | 
 FAEA D3 :|  