Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
M:4/4
K:Bmin
['FB B2', 'bf f2']  
X:1
L:1/8
M:4/4
K:Bmin
 FB B2 f2 ed | cAeA fA e2 | FBBc d2 cd | ec c2 ABcA | FB B2 fedf | efec ABcA | dcBd cBAB | 
 FBBA B4 :| bf f2 fece | a2 ea faea | bf f2 fecA | BdcA B4 | bf f2 fece | a2 ea faec | dcBd cBAB | 
 FBBA B4 :|  