Question: Change the value of a variable, that I have already declared in a proc

Hi! I need a help to understand how the variables in maple 17 works

I want to know if it's possible to change a variable, that I have already declared in a proc, along the body of a procedure. For example this procedure:

>Bucklingload:= proc(a,b,c,d,Q,pr)
local A;
A:=FN(a,b,c,d,Q); #vector of another gobal variable
while A[1] >0 do
Q:=Q+pr ;#"Q"and "pr" are both a list, wich the first value of the vector B decrease his value after an increase estimated in "pr" in "Q"
A[1];
end do;
end proc;

After execute the worksheet, maple display this message:" illegal use of a formal parameter ". I want to know if its possible to use Q as a local variable chaging his value in this loop

Thank you all and sorry for my english!