Question: how to determine unique symbol to use inside a proc?

What is a good way to determine, at run time, a free symbol to use inside a proce, so it does not appear the same as one being passed in from outside?

Here is the problem. Inside a proc, user calls it with an ODE. The independent variable is passed in as the argument called x

Inside this proc, it does change of variables to do something else.  If the proc uses say t as the new variabe, this could be the same as the passed in argument in the way it appears when being printed out to file.  

I know the local symbol t is not the same as the passed in symbol, even if that happend to be t also. As one is local and the other is not. So the math will still work, but the display will look very confusing to the user.

But since I am printing these out, these will show the same. 

So I need a way to make symbol inside proc, which looks different from the one being passed in.

An example will show the problem. 

foo:=proc(ode::`=`,y,x)
local t; #hoping this will not be the same looking as actual x
return PDEtools:-dchange({x=t^2},ode,t)  
end proc;

Now it is called as

ode:=diff(y(x),x)=sin(x);
foo(ode,y,x)

Which gives

The above worked, because the independent variable being passed in happend to be x

But if the proc was next called using global t as the independent variable

ode:=diff(y(t),t)=sin(t);
foo(ode,y,t)

The output will be the same. Which looks confusing.

I want the out in this case to be using different letter than t, say z

The point is, the proc needs to use different looking symbol. Since it does not know which symbol the user is using, currently I use Z  since this is not likely to be used as independent variable by a user. most odes use x or t or z as independent variables

Again, this is all for display purposes, as I am printing these out, and want to avoid using the same looking symbol.

What is a good way to come up with symbol that does not look like the one being passed in?   The option of the user passing in the extra variable to use for this is not possible at all, as this is done deep inside and asking the user to pass in the spare symbol to use is not possible.