pub const IDENTITY2: [f64; 9];
Second-order identity tensor in Mandel basis (I)
Mandel vector: ┌ ┐ │ 1 │ │ 1 │ │ 1 │ │ 0 │ [I] = │ 0 │ │ 0 │ │ 0 │ │ 0 │ │ 0 │ └ ┘