- Using sequences: https://github.com/sympy/sympy/wiki/UD-Sequences-and-formal-power-series-prototype

This is restricted to one variable only. Then, there was an attempt using series by formula. - Using ring_series: bases on polynomials, like you said
- Using dictionaries: that we discussed and you implemented in a.py

a.py is faster than b.py and can be extended to multivariate series using Kronecker's substitution. Do we need to extend ring_series?

I posted a query regarding this on the mailing list : https://groups.google.com/forum/#!topic/sympy/TVEp72mZ3Uo