|   | 1 |  initial version  | 
One way to obtain x + f(x) is as follows:
sage: import sympy
sage: f = sympy.Function('f')
sage: g(x) = x
sage: f(x) + g(x)
x + f(x)
I'm not sure how to mix this with your finite field problem.
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                Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.