Removing assignment at the top-level



Dear All ---

 How do I remove assignments, once having bound a variable at the
top-level ?

 Look at this:


(%i16)  z : 9 - sqrt(z)^2;
(%o16)                               9 - z
(%i17)  z : 9 - (z)^2;
                                            2
(%o17)                           9 - (9 - z)
(%i18)  z : 9 - z^2;
                                              2 2
(%o18)                        9 - (9 - (9 - z) )
(%i19)  z : (9 - z)^2;
                                            2 4
(%o19)                          (9 - (9 - z) )
(%i20) z : 0$
(%i21) z;
(%o21)                                 0
(%i22)  z : 9 - z^2;
(%o22)                                 9
(%i23)  z := 9 - z^2;

 I couldn't find in the documentation. Perhaps I need to sleep?
 How do I "forget"?

 TIA.

 Henry