Construct a musical composition following the given alphabetical structure and incorporating the specified motif.
X:1
L:1/8
M:4/4
K:C
['_B c2 GC']  
X:1
L:1/8
M:4/4
K:C
 _BG | BG _E=E C C3 | z2 z _B c2 GC | EC EC C4- | C4 z2 | _BG | B3 cc CC C | c/c/ _B c2 GC C_E | 
 C8 | z4 z2 EE | F F/F/ F F/F/ F G2 G | _B/B/ B G/B/ E/C/ C/C/ C C2- | C8 | z4 ||  