How do I get the result of right_kernel into a matrix?

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Hello, I have a simple program I am running where I define a matrix C, then define

B=C.right_kernel()

I would then like to create another matrix, D, that is a block matrix with C and B as blocks, C on top of B. So far my code for that is

Dlist = []

for i in range(len(rowsofmatrix)):

Dlist.append(C[i,:])

for i in range(20-len(rowsofmatrix)):

Dlist.append(B[i,:])

where rowsofmatrix is a list I used to define the matrix C. The Dlist.append(C[I,:]) works just fine, but the second one involving B does not, and I imagine this has something to do with the form of B. Is there a way I can express B as a regular matrix? Or, alternatively, is there a better way to do this altogether?

Thanks