class SyncControl
    Model of synchronous control state value.
    terminated: Not used.
    blocked: out of fthreads to run.
    delegated: non synchronous service call, delegate to our caller.
    enum fstate_t = terminated, blocked, delegated;
    instance Str[fstate_t] {
      fun str: fstate_t -> string = 
      | #terminated => "terminated"
      | #blocked => "blocked"
      | #delegated => "delegated"
    Fthread state model.
    next_fthread_pos: restart scheduler by grabbing a new fthread
    from the active list.
    next_request_pos: restart scheduler by handling the service
    request of the currently active fthread.
    enum fpc_t = next_fthread_pos, next_request_pos;
    The type of a list of active fthreads to schedule.
    type fthread_list = "::std::list<::flx::rtl::fthread_t*>*" requires Cxx_headers::list;
    Construct an empty fthread list.
    ctor fthread_list: 1 = "new ::std::list<::flx::rtl::fthread_t*>";
    Delete an fthread list.
    proc delete : fthread_list = "delete $1;";
    Push an fthread onto an fthread list.
    proc push_back: fthread_list * fthread = "$1->push_back($2); PTF gcp->collector->add_root($2);";
    Upcast a procedure closure to a continuation.
    ctor cont: (1->0) = "static_cast<::flx::rtl::con_t*>($1)";
    Create an fthread from a continuation.
    ctor fthread: cont = "new (*PTF gcp, ::flx::rtl::_fthread_ptr_map, false) ::flx::rtl::fthread_t($1)";
    Type of a synchronous scheduler.
    type scheduler = "::flx::run::sync_sched*" requires header '#include "flx_sync.hpp"';
    Constructor for a synchronous scheduler.
    Accepts a list of fthreads to run.
    Uses the callers garbage collector.
    ctor scheduler : fthread_list = "new ::flx::run::sync_sched(false, PTF gcp, $1)";
    proc delete: scheduler = "delete $1;";
    Invoke the scheduler and run until one of two
    events transpires: blocked, delegated.
    proc run : scheduler = "$1->frun();";
    get the exit state of the scheduler after running.
    fun get_scheduler_state : scheduler -> fstate_t = "$1->fs";
    Run a single procedure as a fibre on
    new scheduler. This scheduler acts like a subroutine,
    control doesn't return to the caller until the scheduler
    queue is empty. This call cannot handle async I/O,
    so any attempt to do it will cause the program to abort.
    proc run_proc (p: 1->0) {
      var clsa = cont p;                       // cast procedure to continuation (throw out type)
      var fthr = fthread clsa;                 // construct an fthread from the continuation.
      var lst = #fthread_list;                 // make scheduler list for scheduler to use.
      var sched = scheduler lst;               // construct scheduler.
      push_back (lst,fthr);                    // add our job to the scheduler list.
      run sched;                               // run the scheduler.
      var result = get_scheduler_state sched;  // get return state of scheduler.
      match result with 
      | #terminated => assert false;            // unused.
      | #delegated =>                           // bug out if async I/O attempted.
        raise "Inner scheduler attemped unavailable service call";
      | #blocked =>                             // this scheduler ran out of threads.
        delete sched;                          // cleanup: delete the scheduler
        delete lst;                            // and the scheduler list.