### solve, indexes of solution variables are incrementing

Hi

SageMath 8.6 notebook with W10 and Edge.

why indexes of solution variables are incrementing each time I execute the cell, despite I put a forget() at the beginning?
(on the cocalc site, you must add or delete a space and then click on run to see the incrementation of the variables from the solution of the vector U),first r_1,r_2 then r_3,r_4 ... etc.
if I reset the kernel then it start again at r_1,r_2 ~~despite I put a forget() at the beginning.
~~.
On Cocalc : Solve matrix N*U=M

it's not embarrassing, but it puzzles me ;-)