api: fix substitution when derivative appears multiple times

Author: mloubout

devito/types/equation.py

@@ -98,7 +98,7 @@ def _apply_coeffs(cls, expr, coefficients):
         if not mapper:
             return expr
 
-        return expr.xreplace(mapper)
+        return expr.subs(mapper)
 
     def _evaluate(self, **kwargs):
         """

examples/seismic/tutorials/07_DRP_schemes.ipynb

@@ -83,7 +83,7 @@
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "Eq(-u(t, x, y)/dt + u(t + dt, x, y)/dt + (0.1*u(t, x, y) - 0.6*u(t, x - h_x, y) + 0.6*u(t, x + h_x, y))/h_x, 0)\n"
+    "Eq(-u(t, x, y)/dt + u(t + dt, x, y)/dt + 0.1*u(t, x, y)/h_x - 0.6*u(t, x - h_x, y)/h_x + 0.6*u(t, x + h_x, y)/h_x, 0)\n"
      ]
     }
    ],

tests/test_symbolic_coefficients.py

@@ -202,8 +202,8 @@ def test_staggered_equation(self):
 
         eq_f = Eq(f, f.dx2(weights=weights))
 
-        expected = 'Eq(f(x + h_x/2), (1.0*f(x - h_x/2) - 2.0*f(x + h_x/2)'\
-            ' + 1.0*f(x + 3*h_x/2))/h_x**2)'
+        expected = 'Eq(f(x + h_x/2), 1.0*f(x - h_x/2)/h_x**2 - 2.0*f(x + h_x/2)/h_x**2 '\
+            '+ 1.0*f(x + 3*h_x/2)/h_x**2)'
         assert(str(eq_f.evaluate) == expected)
 
     @pytest.mark.parametrize('stagger', [True, False])