Classify the key-value pairs inside poly according to levels of summation
Parameters:
poly (Poly) – poly with the first part of its keyfor =
(‘e^{-kt}’,’t’,’k^{-}’,’v0-theta’,’theta’,’sigma’,
‘l_{1:n}’, ‘o_{1:n}’)
and the second part may be (‘sigma/2k’,’rho-sigma/2k’,’sqrt(1-rho^2)’)
or (‘rho’)