Merge pull request #303 from isuruf/is

Use is instead of == for singletons

Author: isuruf

symengine/lib/symengine_wrapper.pyx

@@ -433,9 +433,9 @@ def sympy2symengine(a, raise_error=False):
         return piecewise(*(a.args))
     elif isinstance(a, sympy.Interval):
         return interval(*(a.args))
-    elif a == sympy.S.EmptySet:
+    elif a is sympy.S.EmptySet:
         return emptyset()
-    elif a == sympy.S.UniversalSet:
+    elif a is sympy.S.UniversalSet:
         return universalset()
     elif isinstance(a, sympy.FiniteSet):
         return finiteset(*(a.args))