Compose a piece of music that follows both the given musical theme and the letter-based structure provided.
X:1
L:1/4
M:6/4
K:D
['f g', 'c B']  
X:1
L:1/4
M:6/4
K:D
 D3 F2 A | d3 f a f | e2 d c2 B | A3- A3/2 e/f/g/ | a b f g b e | f g d e a g/ | f3/2 e/ d A F A | 
 d3- d2 :| e3 d3 | c B A c B A | f3 e3 | d c B d c B | g3 f3 | e f g d e f | c3/2 B/ A e a ^g | 
 a3- a2 f | g2 b a3/2 g/ f | (e a2) (e g2) | f e d f e d | c3 A3/2 e/f/g/ | a2 A G2 g | 
 f2 F E A G | F3/2 E/ D A d c | d3 (A B) =c | B A G B A G | A3 G3 | F3/2 E/ D A,2 C | D3- D2 ||  