function subtract(out, a, b) {
    out[0] = a[0] - b[0];
    out[1] = a[1] - b[1];
    out[2] = a[2] - b[2];
    out[3] = a[3] - b[3];
    return out;
  }
Subtracts matrix b from matrix a

@param {mat2} out the receiving matrix
@param {mat2} a the first operand
@param {mat2} b the second operand
@returns {mat2} out