You can do all of these as macros (etc) rather than a compiler. For example, an `fn' macro would inspect the body and plant `nil' return values; for arbitrary `='s see swindle's `setf!', the convenient indexing is easily done with generalized functions, rest notation and destructuring also exist in plt and would be easy to incorporate. The difficulties lie more in the subtle points: for example, plt macros have phase separation, and arc is more lispish with a single phase for everything -- and doing that would be very difficult at best. As another example, mutable cons pairs are hacked onto arc (when using the newer plt versions) in a way that works out because arc is essentially serializing its own lists -- but if you use more plt-isms, and end up using more consed lists, you are more likely to run into problems with stepping over the plt assumption of pairs being immutable.
