mysum(e, v, lo, hi) := block([s: 0], for i from lo thru hi do s: s + subst(v=i, e), s)$

mysum(1/n, n, 1, 10);
7381/2520

/* compare with builtin sum */
sum(1/n, n, 1, 10);
7381/2520

/* what if n is assigned a value ? */
n: 200$

/* still works */
mysum(1/n, n, 1, 10);
7381/2520