user@host:~/koan-tracts$ make
KLEE: ERROR: /home/user/koan-tracts/additive_koan.c:33: ASSERTION FAIL: return_ok
COUNTEREXAMPLE: 
object    1: name: 'a_line_28'
object    1: size: 1
object    1: data: '\x00'
object    2: name: 'b_line_29'
object    2: size: 1
object    2: data: '\x80'
make: *** [additive_koan.ok] Error 3
rm additive_koan.bc