Pass substitution rules to substitution rules?

[inducer]

cc @a-alveyblanc

Example:

dot(a, b, n):=domain({[i]: 0<=i<n}, sum(i, a(i), b(i)))

Or something:

dot($a, $b, n):=domain({[i]: 0<=i<n}, sum(i, $a(i), $b(i)))

[a-alveyblanc]

Should be easy to add support for this on top of #970