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: coordsystem3dbuilder.rs
    path of file: ./repos/rnote/crates/rnote-compose/src/builders
    
    the code of the file until where you have to start completion: // Imports
use super::buildable::{Buildable, BuilderCreator, BuilderProgress};
use crate::eventresult::EventPropagation;
use crate::penevent::{PenEvent, PenState};
use crate::penpath::Element;
use crate::shapes::Line;
use crate::style::{indicators, Composer};
use crate::{Constraints, EventResult};
use crate::{Shape, Style};
use p2d::bounding_volume::{Aabb, BoundingVolume};
use piet::RenderContext;
use std::time::Instant;

/// 3D coordinate system builder.
#[derive(Debug, Clone)]
pub struct CoordSystem3DBuilder {
    /// Tip of the z axis.
    tip_z: na::Vector2<f64>,
    /// Tip of the y axis.
    tip_y: na::Vector2<f64>,
}

impl BuilderCreator for CoordSystem3DBuilder {
    fn start(element: Element, _now: Instant) -> Self {
        Self {
            tip_z: element.pos,
            tip_y: element.pos,
        }
    }
}

impl Buildable for Coor
    
    pub fn state_as_lines(&self) -> Vec<Line> {
        let center = na::vector!(self.tip_z.x, self.tip_y.y);
        let tip_x_offset =
            ((center - self.tip_z).magnitude() + (center - self.tip_y).magnitude()) / 4.0;

        let up_axis = Line {
            start: center,
            end: self.tip_z,
        };

        let down_axis = Line {
            start: center,
            end: na::vector![center.x, 2.0 * self.tip_y.y - self.tip_z.y],
        };

        let right_axis = Line {
            start: center,
            end: self.tip_y,
        };

        let left_axis = Line {
            start: center,
            end: na::vector![2.0 * self.tip_z.x - self.tip_y.x, center.y],
        };

        let forward_axis = Line {
            start: center,
            end: center + na::vector![-tip_x_offset, tip_x_offset],
        };

        let backward_axis = Line {
            start: center,
            end: center + na::vector![tip_x_offset, -tip_x_offset],
        };

        vec![
            up_axis,
            down_axis,
            right_axis,
            left_axis,
            forward_axis,
            backward_axis,
        ]
    }