Problems with subs

We do not understand how subs works in some cases: we tried the following ginsh session

p=x+1; 1+x subs(p,x==k-1); k subs(p,x==x-1); -1+x subs(p,x==x-2); -1+x subs(p,x==x^2-1); x^2 subs(p,x==x^2); 1+x^2

so, apparently, subs is sometimes wrong. We couldn't find anything in the documentation to explain this fact. Thank you very much. Best wishes Alessandro Zaccagnini Tatiana Zolo purrs developers, at the University of Parma, Italy