Provide a default isapprox method for ring elements

src/Rings.jl

@@ -8,6 +8,21 @@ function isequal(a::RingElem, b::RingElem)
    return parent(a) == parent(b) && a == b
 end
 
+# Implement `isapprox` for ring elements via equality by default. On the one
+# hand, we need isapprox methods to be able to conformance test series rings.
+# On the other hand this is essentially the only sensible thing to do in
+# positive characteristic so we might as well do it in a generic method.
+function Base.isapprox(x::NCRingElem, y::NCRingElem;
+                       atol::Real=0, rtol::Real=0,
+                       nans::Bool=false, norm::Function=abs)
+  if is_exact_type(typeof(x)) && is_exact_type(typeof(y))
+    @req is_zero(atol) "non-zero atol not supported"
+    @req is_zero(rtol) "non-zero rtol not supported"
+    return x == y
+  end
+  throw(NotImplementedError(:isapprox, x, y))
+end
+
 """
     divexact(x, y; check::Bool=true)