Trac #17402: SR.power_series cannot handle symbolic series

{{{
sage: x=var('x')
sage: s=(1/(1-x)).series(x,6)
sage: ps=s.power_series(SR)
sage: ps
x^5 + x^4 + x^3 + x^2 + x + Order(x^6) + 1 + O(x)
sage: ps=s.power_series(QQ)
...
TypeError: unable to convert 1 + 1*x + 1*x^2 + 1*x^3 + 1*x^4 + 1*x^5 +
Order(x^6) to a rational
}}}
This is analogous to #16203 and can be worked around using `truncate()`
but really should work out of the box.

URL: http://trac.sagemath.org/17402
Reported by: rws
Ticket author(s): Ralf Stephan
Reviewer(s): Volker Braun

src/sage/symbolic/series.pyx

@@ -232,3 +232,21 @@ cdef class SymbolicSeries(Expression):
                 ret[c[1]] = c[0]
             return ret
 
+    def power_series(self, base_ring):
+        """
+        Return algebraic power series associated to this symbolic
+        series. The coefficients must be coercible to the base ring.
+
+        EXAMPLES::
+
+            sage: ex=(gamma(1-x)).series(x,3); ex
+            1 + (euler_gamma)*x + (1/2*euler_gamma^2 + 1/12*pi^2)*x^2 + Order(x^3)
+            sage: g=ex.power_series(SR); g
+            1 + euler_gamma*x + (1/2*euler_gamma^2 + 1/12*pi^2)*x^2 + O(x^3)
+            sage: g.parent()
+            Power Series Ring in x over Symbolic Ring
+        """
+        from sage.rings.all import PowerSeriesRing
+        R = PowerSeriesRing(base_ring, names=str(self.default_variable()))
+        return R(self.list(), self.degree(self.default_variable()))
+