== step =====
step =>> fb1.step   (*  :  *)
  fb1.stepO =>> fb2.step   (*  :  *)
    fb2.stepO =>> stepO   (*  :  *)
== ctor =====
ctor =>> fb1.ctor   (*  :  *)
== init =====
init =>> fb1.init   (*  :  *)
init =>> fb2.init   (*  :  *)
