1 | initial version |

See

```
sage: A[1]
(1, 4, 6)
sage: A[1].is_immutable()
True
```

You get a vector that is immutable. When you do

```
sage: A[1][1]=6
```

you try to change the entry of index 1 of that vector, not the matrix `A`

. The matrix `A`

is mutable:

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

To change the entry of index `1,1`

of the matrix `A`

, you should do:

```
sage: A[1,1]=6
```

So that

```
sage: A
[ 1 2 -3]
[ 1 6 6]
[ 2 -1 -2]
```

2 | No.2 Revision |

See

```
sage: A[1]
(1, 4, 6)
sage: A[1].is_immutable()
True
```

You get a vector that is immutable. When you do

`sage: `~~A[1][1]=6
~~A[1][1] = 6

you try to change the entry of index 1 of that vector, not the matrix `A`

. The matrix `A`

is mutable:

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

To change the entry of index `1,1`

of the matrix `A`

, you should do:

`sage: `~~A[1,1]=6
~~A[1,1] = 6

So that

```
sage: A
[ 1 2 -3]
[ 1 6 6]
[ 2 -1 -2]
```

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