def val_from_sp(expression):
    """Transforms a sympy expression to a callable function
    f(x, y)

    Args:
        expression (sympy.Add): sympy expression to be converted
            which has symbols 'x' and 'y' in it.
    """
    def val(x, y):
        return expression.subs('x', x).subs('y', y)
    return val