Design a musical composition incorporating the alphabetic expression of the chosen structure and the provided melodic theme.
X:1
L:1/8
M:6/8
K:C
['gec gec', 'c3 def fdB gec gec def feB']  
X:1
L:1/8
M:6/8
K:C
 gec gec | a2 g g3 | gec gec | f2 e e3 | gec gec | afc afc | efg gfe | c3 c3 :: def fdB | gec gec | 
 def feB | gec ceg | afa afa | geg geg | agf efd | ccc c3 :|  