You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: mod.rs
    path of file: ./repos/plotters/plotters/src/element
    
    the code of the file until where you have to start completion: /*!
    Defines the drawing elements, the high-level drawing unit in Plotters drawing system

    ## Introduction
    An element is the drawing unit for Plotter's high-level drawing API.
    Different from low-level drawing API, an element is a logic unit of component in the image.
    There are few built-in elements, including `Circle`, `Pixel`, `Rectangle`, `Path`, `Text`, etc.

    All element can be drawn onto the drawing area using API `DrawingArea::draw(...)`.
    Plotters use "iterator of elements" as the abstraction of any type of plot.

    ## Implementing your own element
    You can also define your own element, `CandleStick` is a good sample of implementing complex
    element. There are two trait required for an element:

    - `PointCollection` - the struct should be able to return an itera
    
    fn main() -> Result<(), Box<dyn std::error::Error>> {
        let root =
            BitMapBackend::new("plotters-doc-data/element-3.png", (640, 480))
            .into_drawing_area();
        root.fill(&WHITE)?;
        let mut chart = ChartBuilder::on(&root)
            .x_label_area_size(40)
            .y_label_area_size(40)
            .margin(5)
            .build_cartesian_2d(0..50, 0..10)?;

        chart
            .configure_mesh()
            .disable_x_mesh()
            .disable_y_mesh()
            .draw()?;

        chart.draw_series((0..50).map(|x| {
            let center = (x, num_of_factor(x));
            // Although the arms of if statement has different types,
            // but they can be placed into a dynamic element wrapper,
            // by doing so, the type is unified.
            if center.1 == 2 {
                Cross::new(center, 4, Into::<ShapeStyle>::into(&RED).filled()).into_dyn()
            } else {
                Circle::new(center, 4, Into::<ShapeStyle>::into(&GREEN).filled()).into_dyn()
            }
        }))?;

        Ok(())
    }