A bunch of utility functions

utils.m

zeros[0] = 0;
zeros[dim_] := ConstantArray[0, {dim, dim}];

MF[expr_] := MatrixForm@expr;

(* take small matrix and embed it into a larger matrix, padding with zeros *)
injectIntoLargerMatrix[mat_, largerMatDim_, position_Integer: 1] := Module[{outMat},
  (* put zeros on bottom right *)
  If[largerMatDim - Length@mat - position + 1 > 0,
    outMat = ArrayFlatten[{
      {mat, 0}, {0, zeros[largerMatDim - Length@mat - position + 1]}
    }],
    outMat = mat
  ];
  (* put zeros of upper left *)
  If[position > 1,
    outMat = ArrayFlatten[{
      {zeros[position - 1], 0}, {0, outMat}
    }]
  ];
  outMat
];
(* if `indicesToReplace` is a list, and the second argument a matrix, then we use the third
   argument to know which elements/indices of `largerMat` should be replaced with `mat` *)
injectIntoLargerMatrix[mat_, largerMat_, indicesToReplace_List] := Module[{outMat},
  outMat = largerMat;
  outMat[[Sequence @@ indicesToReplace]] = mat;
  outMat
];