Returns a context dictionary for use in evaluating the expression.

        :param arr: The input array.
        :param expr: The input expression.
        :param context: Evaluation context.

def get_context(self, arr, expr, context):
        """
        Returns a context dictionary for use in evaluating the expression.

        :param arr: The input array.
        :param expr: The input expression.
        :param context: Evaluation context.
        """

        expression_names = [x for x in self.get_expression_names(expr) if x not in set(context.keys()).union(['i'])]

        if len(expression_names) != 1:
            raise ValueError('The expression must have exactly one variable.')

        return {expression_names[0]: arr}