Craft a musical piece that abides by the prescribed structural guidelines and the given theme.
X:1
L:1/8
M:2/4
K:Bb
['F BBBB B2 FF dddd d2 B2']  
X:1
L:1/8
M:2/4
K:Bb
 F | BBBB | B2 FF | dddd | d2 B2 | f2 f2 | fedd | cccc | B2 F |]  