How to use the pymoca.ast.ComponentRef function in pymoca

def get_derivative(self, s):

        # Case 1: s is a constant, e.g. MX(5)
        if ca.MX(s).is_constant():
            return 0

        # Case 2: s is a symbol, e.g. MX(x)
        elif s.is_symbolic():
            if s.name() not in self.derivative:
                if len(self.for_loops) > 0 and s in self.for_loops[-1].indexed_symbols:
                    # Create a new indexed symbol, referencing to the for loop index inside the vector derivative symbol.
                    for_loop_symbol = self.for_loops[-1].indexed_symbols[s]
                    s_without_index = self.get_mx(ast.ComponentRef(name=for_loop_symbol.tree.name))
                    der_s_without_index = self.get_derivative(s_without_index)
                    if ca.MX(der_s_without_index).is_symbolic():
                        return self.get_indexed_symbol(ast.ComponentRef(name=der_s_without_index.name(), indices=for_loop_symbol.tree.indices), der_s_without_index)
                    else:
                        return 0
                else:
                    der_s = _new_mx("der({})".format(s.name()), s.size())
                    # If the derivative contains an expression (e.g. der(x + y)) this method is
                    # called with MX variables that are the result of a ca.symvar call. This
                    # ca.symvar call strips the _modelica_shape field from the MX variable,
                    # therefore we need to find the original MX to get the modelica shape.
                    der_s._modelica_shape = \
                        self.nodes[self.current_class][s.name()]._modelica_shape
                    self.derivative[s.name()] = der_s
                    self.nodes[self.current_class][der_s.name()] = der_s
                    return der_s
            else:
                return self.derivative[s.name()]

def exitExpression(self, tree: ast.Expression):
        if isinstance(tree.operator, ast.ComponentRef):
            op_name = tree.operator.name
        else:
            op_name = tree.operator
        if len(tree.operands) == 1:
            self.xml[tree] = E(
                'operator', name=op_name,
                *[self.xml[c] for c in tree.operands])
        else:
            self.xml[tree] = E(
                'apply', builtin=op_name,
                *[self.xml[c] for c in tree.operands])

def get_derivative(self, s):

        # Case 1: s is a constant, e.g. MX(5)
        if ca.MX(s).is_constant():
            return 0

        # Case 2: s is a symbol, e.g. MX(x)
        elif s.is_symbolic():
            if s.name() not in self.derivative:
                if len(self.for_loops) > 0 and s in self.for_loops[-1].indexed_symbols:
                    # Create a new indexed symbol, referencing to the for loop index inside the vector derivative symbol.
                    for_loop_symbol = self.for_loops[-1].indexed_symbols[s]
                    s_without_index = self.get_mx(ast.ComponentRef(name=for_loop_symbol.tree.name))
                    der_s_without_index = self.get_derivative(s_without_index)
                    if ca.MX(der_s_without_index).is_symbolic():
                        return self.get_indexed_symbol(ast.ComponentRef(name=der_s_without_index.name(), indices=for_loop_symbol.tree.indices), der_s_without_index)
                    else:
                        return 0
                else:
                    der_s = _new_mx("der({})".format(s.name()), s.size())
                    # If the derivative contains an expression (e.g. der(x + y)) this method is
                    # called with MX variables that are the result of a ca.symvar call. This
                    # ca.symvar call strips the _modelica_shape field from the MX variable,
                    # therefore we need to find the original MX to get the modelica shape.
                    der_s._modelica_shape = \
                        self.nodes[self.current_class][s.name()]._modelica_shape
                    self.derivative[s.name()] = der_s
                    self.nodes[self.current_class][der_s.name()] = der_s
                    return der_s