code: 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];
    out[4] = a[4] - b[4];
    out[5] = a[5] - b[5];
    out[6] = a[6] - b[6];
    out[7] = a[7] - b[7];
    out[8] = a[8] - b[8];
    return out;
  }
docs: Subtracts matrix b from matrix a
  
  @param {mat3} out the receiving matrix
  @param {mat3} a the first operand
  @param {mat3} b the second operand
  @returns {mat3} out