A = {{1,1},{1,1}}
B = {{2,2,2},{2,2,2}}
C = {{3,3},{3,3},{3,3},{3,3}}
D = {{4,4,4},{4,4,4},{4,4,4},{4,4,4}}

print("Matrix.Flatten({{A,B},{C,D}})" )
print( Matrix.Flatten({{A,B},{C,D}}) )

print("\n\n\n")
M = Matrix.Zero(3)
M[1][1] = A
M[1][2] = B
M[2][2] = D
M[3][3] = C
print( "Matrix.Flatten(M)" )
print( Matrix.Flatten(M) )