Question: Error, (in maplemint/expression) not implemented POLY

Hello,
When I try to put the Gcdex in a procedure and start maplemint, then there occurs an error.

Gcdex(x^2 - 1, x - 2, x ,'s','t') mod 3;

--> works

But:

restart;
a := proc()
    Gcdex(x^2 - 1, x - 2, x ,'s','t') mod 3;
end proc;
maplemint(a);

Then there is an error I don't understand.

Error, (in maplemint/expression) not implemented POLY

By the way I have a fundamental problem to understand, where the values s and t are saved after calling Gcdex (or Quo, Rem, etc.). Till now I thought, that variables s and t are created, but when I declare s, t at the beginning as local variables and start maplemint, then there is something like:

    These parameters have the same name as constants:
      3
    These local variables were used before they were assigned a value:
      r::name, (-x-1)::name, (x-1)::name

So the names of s and t changed, they don't assign a new value? I don't understand that.