Array operator "sum array values" + matching dimensions

> create or replace function sum_elements(anyarray)
> returns anyelement as $$
> select sum($1[i])
> from generate_series(array_lower($1,1),
> array_upper($1,1)) g(i);
> $$ language sql immutable;

Thank you! Anyway what I was really asking was a "$" (or whatever other symbol) in place of ":" for sum, that is a notation like

myarray_col[12$16]

would give the sum of all values from myarray_col[12] to myarray_col[16]

Anyway, I was reading that multidimensional arrays must have matching dimensions (that is, they can't be, for example, [5][9]).
Is this right? Why is that?

Thank you

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