plasma_in_flight_exit

plasma in-flight exit approach
-  on mainchain user starts exit with tx-data(user signed and submited) and exits with last amount(in erc20) 
-  user also plases a bond amount 
-  there is fraud-prrof if user tries to exit with fraudulent tx then operator can submit in-flight tx with same nonce
-  this prevents fraud from operator side 
-  obviusly user won't exit with fake tx(where he is sending amount to someone) with less amount then his last accepted tx amount
-  if user tries to exit with any previous tx then in-flight tx then operator or anyone with greater nonce tx can strat challenge 
    and get bond amount and cancels exit
-  what happens when user and operator colludes ?