Generate a piece of music.  
X:1
T:Music21 Fragment
C:Music21
L:1/4
M:none
I:linebreak $
K:none
V:1 treble 
V:1
{/D} B, C2 | C E E F | E E B, C | E F G G ||{/G} c2 B2 | G G F E | G E2 F | E2 B, C || %8
{/D} B, C D D | B, C E F | G2 E F | E C2 B, || %12  