As most of you already know

a[i] = v

is actually

a.set(i,v)

I should also note that this transformation happens very early in compiler pipeline. Now think of

t := a[i] = v

set returns this, so t will be assigned to a, not the stored value, as some might expect.

Don't know. Seems logical to me (if you read the docs). What did you expect to return? Bool or the element that was added?

I'd expect to get stored result as in C/Java/whatever, so that:

x = expr = v

is basically the same as

expr = v; x = expr