// storage type
type storage_t is address;

// entry points parameter types
type change_addr_pt is address

type message_t is list (operation) ;
type pass_message_pt is (unit -> message_t)

type contract_return_t is (list(operation) * storage_t)

type entry_point_t is
| Change_address of change_addr_pt
| Pass_message of pass_message_pt

function change_address (const param : change_addr_pt;
                         const s : storage_t) : contract_return_t is
  begin
    if sender =/= s then failwith("Unauthorized sender")
    else skip
end with ((nil : list(operation)), param)

function pass_message ( const param: pass_message_pt;
                        const s : storage_t ) : contract_return_t is
  begin
    if sender =/= s then failwith("Unauthorized sender") else skip ;
  var message : pass_message_pt := param ;
end with (param(unit),s)

function main(const param : entry_point_t; const s : storage_t) : contract_return_t is 
block {skip} with 
  case param of
  | Change_address (p) -> change_address(p,s)
  | Pass_message (p) -> pass_message(p,s)
end