Craft a musical composition that adheres to the specified musical theme and letter-based structure.
X:1
L:1/8
M:4/4
K:C
['', 'G3 F (Gc)', '(.B.c.d) e3 d (dc)(.B.G)']  
X:1
L:1/8
M:4/4
K:C
 !>!dde |][M:4/4][K:Eb][Q:1/4=60] DEF | G3 F (Gc) (BG) | (FE) C2 z E(.G.A) | B3 (c B G)(.E.G) | 
 F4 z D(.E.F) | G3 F (Gc) (.B.G) | (FE.C.=B,) (CDEF) | G3 A (GF)(EF) | E4 z || (.B.c.d) | 
 e3 d (dc)(.B.G) | (BG) E2 z (.B.c.d) | e3 d (dc)(.B.G) | F4 z (.B.B.B) | g3 f (fe) (ce) | 
 (BG) E2 z (D.E.F) | (Gc) (BG) (FE) (.C.D) | (E4 E) |]  