add tests for solved issue sagemath#15971

Author: videlec

src/sage/rings/fraction_field.py

@@ -55,6 +55,18 @@
     sage: F = FractionField(PolynomialRing(RationalField(),2,'x'))
     sage: F == loads(dumps(F))
     True
+
+Test that :trac:`15971` is fixed::
+
+    sage: for B in [QQ['t'], QQ['s, t'], ZZ['t'], ZZ['s, t']]:
+    ....:     F = B.fraction_field()
+    ....:     R = F['x, y']
+    ....:     x = R.gen(0)
+    ....:     print(x / x)
+    1
+    1
+    1
+    1
 """
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