ppl program question

[DrD]

In CAS a series of steps with variables containing expressions works symbolically just fine:

r:=[-sin(t)+t,-cos(t)]
v:=diff(r,t)
a:=diff(v,t)

each step retains its symbolic representation, unevaluated, enabling successive computation to proceed.

However, a program doing the same thing evaluates each step numerically, thereby losing context for further steps:

EXPORT a()
BEGIN
local a,r,t,v;
r:=[t-sin(t),−cos(t)];
v:=diff(r,t);
a:=diff(v,t);
END;

Variable 'r' becomes [0,-1] and the program errors beyond that step. I've tried the quote(Expr) function: "Returns an expression unevaluated," as described in the user guide, which returns 'r'. If possible, how can I make the program behave as if manually entered in cas?

Thanks!

-Dale-