Design a musical composition incorporating the alphabetic expression of the chosen structure and the provided melodic theme.
X:1
L:1/8
M:4/4
K:Bb
['', 'B,>F D<G', 'f<B d>f']  
X:1
L:1/8
M:4/4
K:Bb
 D |: B,>F D<G F<B,{E} D2 | B,>FD>F G/F/E/D/ C<D | B,>F D<G F<B, D>B ||1 F>GB>c d/c/B/A/ B<d :|2 
 F>GB>c d/c/B/A/ B<g || f>B e/d/c/B/ f<B{Bc} d2 | f<B d>f g>ce>g | f<B d>f B>fd>f | e>gd>f g<ce<g | 
 f>B e/d/c/B/ f<B{Bc} d2 | f<B d>f g/f/e/d/ c/B/A/G/ | F<B G/F/E/D/ F<B D>F | (3EGE (3DFD G<CEG ||  