Shared via Pony Playground (https://playground.ponylang.org/?gist=ed4bb2f640b17253216fc861c3dee8c6&branch=regions)

main.pony

class Counter
  var value: U32 = 0
  fun ref increment(delta: Delta ref) =>
    value = value + 1
    delta.diff = delta.diff + 1

class Delta
  var diff: U32 = 0
  
  var foo: Any box = None

actor Main
  new create(env: Env) =>
    var counter : Counter iso = Counter
    var delta : Delta iso = Delta

    counter = recover
      let counter' : Counter ref = consume counter
      
      delta = recover
        let delta' : Delta ref = consume delta
        
        // counter' and delta' are both ref, but in different regions
        // The compiler will ensure they stay separated
        // For example this does not work
        // delta'.foo = counter'
        
        // This is not allowed yet, because method calls assume receiver and all arguments have the same region
        // With some annotations on increment's definition we could specify that we allow them to come from distinct regions
        // counter'.increment(delta')
        
        // Note that for sendable caps regions are ignored.
        // This works
        counter'.value = counter'.value + delta'.diff
        
        delta'
      end

      counter'
    end
    
    // counter and delta have been recovered back to iso, and can be sent away