Swift Beta 4 Updates

Superb list of updates from Apple, here's a summary

• Access control: Private (same file), Internal (same module, default), Public (anyone)
• Improvement to range iteration (stride()). More to come in subsequent releases
• final, lazy, optional and required all lose the @ prefix, more to come later
• Comment "landmarks" //TODO: //MARK: //FIXME: also listed in the jump bar
• Fixes to unicode string iteration (phew)
• Complete Swift language supported in REPL

There's some more, but these are the real highlights. I have to be honest, best update yet. Love how this language is evolving and maturing in front of us.