function definition



Hello,
  I don't understand what is happening in the following lines.

(%i9) S(f,x):= diff(f(x),x);
(%o9)                      S(f, x) := diff(f(x), x)
(%i10) f(x):= x^3;
                                            3
(%o10)                            f(x) := x
(%i11) S(f,x);
                                         2
(%o11)                               3 x
(%i12) g(x):= sin(x);
(%o12)                          g(x) := sin(x)
(%i13) S(g,x);
                                         2
(%o13)                               3 x
(%i14) kill(f);
(%o14)                               done
(%i15) S(g,x);
(%o15)                              cos(x)


Why did I have to do 'kill(f)' to get S(.,x) to operate on g ?

TIA,
  -sen

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