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R.<x> = PolynomialRing(GF(7)['y,z'])

R
Univariate Polynomial Ring in x over Multivariate Polynomial Ring in y, z over Finite Field of size 7 
R.rename('R')

R
R 
R.rename("F7[y,z][x]")

R
F7[y,z][x] 
MatrixSpace(R,2)
Full MatrixSpace of 2 by 2 dense matrices over F7[y,z][x] 
x.rename('foo')
Error in lines 1-1 Traceback (most recent call last): File "/cocalc/lib/python2.7/site-packages/smc_sagews/sage_server.py", line 1044, in execute exec compile(block+'\n', '', 'single', flags=compile_flags) in namespace, locals File "", line 1, in <module> File "sage/structure/sage_object.pyx", line 173, in sage.structure.sage_object.SageObject.rename (build/cythonized/sage/structure/sage_object.c:2470) raise NotImplementedError("object does not support renaming: %s" % self) NotImplementedError: object does not support renaming: x 
x
x