#4835 simptimes: sumexpand appends the %sum factor without ordering

[David Scherfgen]

Spotted by Claude:

s: 'sum(f(i),i,1,n)$
e1: z*s$              /* sumexpand=false: ((mtimes) sum z) — canonical */
sumexpand: true$
e2: z*s$              /* ((mtimes) z sum) — non-canonical */
is(e1 = e2);          /* false, should be true */