symIndex -- sort the components of a Sequence

We use this function when we use variables which are indexed by a sequence of integers which we view as being symmetric in the components.

i1 : symIndex(a,d,c,b)

o1 = (a, b, c, d)

o1 : Sequence

i2 : symIndex(3,1,2)

o2 = (1, 2, 3)

o2 : Sequence