code: function adjoint(out, a) {
    // Caching this value is nessecary if out == a
    var a0 =  a[0]
    out[0] =  a[3]
    out[1] = -a[1]
    out[2] = -a[2]
    out[3] =  a0
  
    return out
  }
docs: Calculates the adjugate of a mat2
  
  @alias mat2.adjoint
  @param {mat2} out the receiving matrix
  @param {mat2} a the source matrix
  @returns {mat2} out