change _process_symbol() to use create_copy() not _binary_create_copy().

pybamm/expression_tree/symbol.py

@@ -985,10 +985,12 @@ def new_copy(
         Optionally gives the copied symbol new children.
 
         If `perform_simplifications` = True, some classes (e.g. `BinaryOperator`,
-        `UnaryOperator`, `Concatenation`) will perform simplifications and checks
+        `UnaryOperator`, `Concatenation`) will perform simplifications and error checks
         based on the new children before copying the symbol. This may result in a
-        different symbol being returned than the one copied. This behaviour can be
-        turned off if the symbol is required to remain unchanged.
+        different symbol being returned than the one copied.
+
+        Turning off this behaviour to ensure the symbol remains unchanged is
+        discouraged.
         """
         obj = self.create_copy(new_children, perform_simplifications)
         obj._print_name = self.print_name

pybamm/parameters/parameter_values.py

@@ -712,10 +712,6 @@ def _process_symbol(self, symbol):
             # Process again just to be sure
             return self.process_symbol(function_out)
 
-        elif isinstance(symbol, pybamm.BinaryOperator):
-            new_children = [self.process_symbol(child) for child in symbol.children]
-            return symbol._binary_new_copy(new_children[0], new_children[1])
-
         # Unary operators
         elif isinstance(symbol, pybamm.UnaryOperator):
             new_child = self.process_symbol(symbol.child)
@@ -740,9 +736,11 @@ def _process_symbol(self, symbol):
                     new_symbol.position = new_symbol_position
             return new_symbol
 
-        # Functions & Concatenations
-        elif isinstance(symbol, pybamm.Function) or isinstance(
-            symbol, pybamm.Concatenation
+        # Functions, BinaryOperators & Concatenations
+        elif (
+            isinstance(symbol, pybamm.Function)
+            or isinstance(symbol, pybamm.Concatenation)
+            or isinstance(symbol, pybamm.BinaryOperator)
         ):
             new_children = [self.process_symbol(child) for child in symbol.children]
             return symbol.create_copy(new_children)