open ABP
  ops s0 s1 s2 s3 : -> Sys .
--> init
  red fifo1(init) .
-->  s1 = rec2(dup1(drop1(send1(send2(send1(init)))))) .
  eq s1 = rec2(dup1(drop1(send1(send2(send1(init)))))) .
  red fifo1(s1) .
--> s2 = rec1(rec1(send2(s1)))
  eq s2 = rec1(rec1(send2(s1))) .
  red fifo1(s2) .
--> s3 = rec2(rec2(rec2(dup1(send1(s2)))))
  eq s3 = rec2(rec2(rec2(dup1(send1(s2))))) .
  red fifo1(s3) .
close