Create a composition by utilizing the given alphabet-based musical pattern and the provided melodic theme.
X:1
L:1/8
M:6/8
K:Bb
['B2 b B3 B2', 'f2 b f2']  
X:1
L:1/8
M:6/8
K:Bb
 B2 b B3 | B2 b B3 | B2 c dcB |1 BAB cAF :|2 FGA B2 |: F | BcB B2 d | fgf f2 b | f2 b f2 e | 
 dec B2 :|  