class Probatio::Chronometer

def record(ev)
  # compute ev.leave_delta if ev is a "leave"
  if ev.enter?
    (@enters ||= []) << ev
  elsif ev.leave?
    e = @enters.pop
    fail "ev mismatch #{ev.name} vs #{e.name}" \
      if ( ! e) || (ev.type != e.type)
    ev.leave_delta = ev.tstamp - e.tstamp
  end
end