Re: What is missing for the seq/string types to be not nil by default?

[Araq]

Optimizations aside, `system.add` and `setLen` need to be able to handle a 
`nil` seq and all the toString operations need to treat `nil` as the empty 
sequence. Note that `len` already returns 0 for nil. `isNil` needs to produce a 
compilation error for seqs and strings. The compiler should not allow to pass 
nil to strings/seqs. So ... internally there would still be nil, but it 
wouldn't be exposed. Implementation improvements could come later.