subscripts in CAS?

[compsystems]

examples of use on CAS MODE, n>0

0: with Σ cmd
purge(a);
Σ(a[k]*x^k,k,1,3) -> x*a[1]+x^2*a[2]+x^3*a[3] // with [] at(a, k-1) == a[1]
Σ(a(k)*x^k,k,1,3) -> x*a(1)+x^2*a(2)+x^3*a(3) // with ()
a:=MAKELIST(x,x,1,3); -> {1,2,3}
Σ(a(k)*x^k,k,1,3) -> x+2*x^2+3*x^3

1: with SUM cmd
purge(a);
sum(a[k]*x^k,k,1,3); -> x*a[1]+x^2*a[2]+x^3*a[3] // with []
sum(a(k)*x^k,k,1,3); -> x*a(1)+x^2*a(2)+x^3*a(3) // with ()
a:=MAKELIST(x,x,1,3); -> {1,2,3}
sum(a(k)*x^k,k,1,3) -> x+2*x^2+3*x^3

2: with SEQ cmd
purge(a);
seq(a(k)*x^k,k,1,3); -> [x*a(1),x^2*a(2),x^3*a(3)]
a:=MAKELIST(x,x,1,3); -> {1,2,3}
seq(a(k)*x^k,k,1,3); ->[x,2*x^2,3*x^3]
ΣLIST( seq(a(k)*x^k, k,1,3)) -> x+2*x^2+3*x^3

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I would like to see the notation using the character ↓ To differentiate it from other operations with parenthesis (evaluation) and so that pretty-print can decode more easily

seq(a↓(k)*x^k,k,1,5); ->
[x*a↓(1),x^2*a↓(2),x^3*a↓(3),x^4*a↓(4),x^5*↓a(5)]