First, let me explain how to deal with lists instead of sets, because there is a trick with sets:

You an create your matrices as follows:

```
sage: a = matrix(ZZ,[[1,2],[3,4]])
sage: b = matrix(ZZ,[[5,6],[7,8]])
sage: c = matrix(ZZ,[[1,2],[3,4]])
sage: d = matrix(ZZ,[[0,1],[1,0]])
```

Then, you can create the list of matrices as follows:

```
sage: L = [a,b,c]
sage: L
[
[1 2] [5 6] [1 2]
[3 4], [7 8], [3 4]
]
```

You can define the other list with *list comprehension*, which is a nice feature of Python:

```
sage: d = matrix(ZZ,[[0,1],[1,0]])
sage: [d*i for i in L]
[
[3 4] [7 8] [3 4]
[1 2], [5 6], [1 2]
]
```

Now, you noticed that `c`

is equal to `a`

and since you want a set, not a list, you would like Sage to keep only one occurence in the set.

You should be able to define a set as follows:

```
sage: S = {a,b,c}
TypeError: mutable matrices are unhashable
```

The error message tells you that the matrices must be immutable to enter a set. Indeed:

```
sage: a.is_mutable()
True
```

So, you can make your matrices immutable first as follows:

```
sage: a.set_immutable()
sage: b.set_immutable()
sage: c.set_immutable()
sage: d.set_immutable()
```

Then you can define your set:

```
sage: S = {a,b,c}
sage: S
{[1 2]
[3 4], [5 6]
[7 8]}
```

As you can see, there are only to elements in it:

```
sage: len(S)
2
```

Now, you shoud be able to define yous second set by comprehension as we did with lists:

```
sage: {d*i for i in S}
TypeError: mutable matrices are unhashable
```

You get the same error. Indeed, when we construct a new matrix, it is mutable by default, and there is no way to construct an immutable one from the beginning (at least this is not documented). So, even if `a`

and `d`

are immutable, `d*a`

is mutable. I agree that this is not very handy.

A workaround is to define a `immutabilize`

function that returns an immutable copy of a mutable matrix:

```
sage: def immutabilize(m):
....: M = copy(m)
....: M.set_immutable()
....: return M
```

So, at the end you get:

```
sage: {immutabilize(d*i) for i in S}
{[3 4]
[1 2], [7 8]
[5 6]}
```