What is the best way to access and modify a column of a matrix inside a taichi kernel?

I wrote some helper function for this task:

```
@ti.func
def getCol(mat, idx):
ret = ti.Vector([0.,0.,0.])
for i in ti.static(range(3)):
ret[i] = mat[i, idx_by_value__]
return ret
@ti.func
def setCol(mat, idx, vec):
assert mat.n == len(vec)
for i in ti.static(range(3)):
mat[i, idx_by_value__] = vec[i]
return mat
```

Here is the test script:

```
M = ti.Matrix.field(3, 3, dtype=ti.f32, shape=(1,1))
M[0,0] = ti.Matrix([
[0,1,2],
[3,4,5],
[6,7,8]
])
@ti.kernel
def test():
print(getCol(M[0,0], 1))
print(setCol(M[0,0], 2, [8,5,2]))
test()
```

If I use the parameter ‘idx’ directly, it will catch an error that says “The 1-th index of a Matrix/Vector must be a compile-time constant integer, got <class ‘taichi.lang.expr.Expr’>.”

However, with this magic variable ‘{paramName}*by_value*_’, these two function works as expected.

I know it can’t be a long-term solution, but are there other ways to do this?

Thank you!