Function concatenate 

pub fn concatenate(arrays: &[Array], axis: impl Axis) -> Array

arrays joined along axis (MLX’s concatenate); dtypes are promoted.

use nuvec_core::{array, ops::concatenate};

let c = concatenate(&[array![[1.0, 2.0]], array![[3.0, 4.0]]], 0);
assert_eq!(c.shape(), [2, 2]);