Develop a musical piece incorporating the designated alphabet-based pattern and the supplied melodic theme.
X:1
L:1/4
M:6/4
K:G
['_B2 c d2']  
X:1
L:1/4
M:6/4
K:G
 A | G2 A _B2 =F | _B2 c d2 d | =f2 d f2 g | d2 A d2 _e | g2 _e _B2 G | _B2 c d2 G | _B2 c d2 G | 
 c2 _B A2 |]  