Write poly into different subpolys according to levels of summation
poly (Poly) – poly with keyfor = (‘e^{kt}’,’t’,’k^{-}’,’v0-theta’,’theta’,’sigma’, ‘l_{1:n}’, ‘o_{1:n}’, ‘sigma/2k’,’rho-sigma/2k’,’sqrt(1-rho^2)’)
keyfor
nth (int) – order of the moment
nothing
None