Craft a musical piece that abides by the prescribed structural guidelines and the given theme.
X:1
L:1/8
Q:3/8=120
M:6/8
K:D
['a2 f faf b2 g', 'FGA AFA'  
X:1
L:1/8
Q:3/8=120
M:6/8
K:D
"_Barred 6/8 in MS, in Gmaj" a2 f faf b2 g | g2 e efd cBA | a2 f faf b2 g | faf gec d3 :: 
 FGA AFA BGB | g2 e efd cBA | FGA AFA B2 g | faf gec"^cr" d3 :|  