Substituting into power series over an inexact ring using keywords fails

[sagetrac-mraum]

sage: R.<x> = CC[[]]
sage: x.subs(x = x**2)
0.0... + 1.0... x

is an example.

The reason is that the generic method subs iterates the generators of R (in
this case x) and checks whether any keyword matches the string
representation of that generator. But

sage: x._repr_()
0.0... + 1.0... x

See also http://groups.google.com/group/sage-devel/browse_thread/thread/c4f80c47b75ffdc8 .

Component: commutative algebra

Issue created by migration from https://trac.sagemath.org/ticket/13227