Utilizing the provided musical theme and letter-based structure, compose a piece of music.
X:1
L:1/8
M:4/4
K:G
['Be dB AG']  
X:1
L:1/8
M:4/4
K:G
 FGA | B3 A Be dB | AG E3 GBc | d3 e dB GB | A4 z FGA | B3 A Be dB | AG E3 FGA | B3 c BA GA | 
 G4 z def | g3 f fe de | dB G3 def | g3 f fe dB | A4 z ded | b3 a ag eg | dB G3 FGA | Be dB AG EF | 
 G4 z |]  