add postcondition about return value and

prove it temporarily with admit
fixe "proof"
remove assertion and simplify proof
adapt proof to new Peek
enhance contract according to Peek
temporarily admit proof obligation of PeekNext
remove an old admit
some progress on proof of PeekNext
improved proof attempt for peeknext
add @ to clean target
prepare lemma
minor
minor
modify driver
put lemma in comment
put mappings in driver file
simplify proof
checkin in between
use old names
replace two admits by application of shift_zero