Running bule2 with introduction examples creates infinite loop.

[StephanieGouw]

when running bule2 --solve --models 0 graph_colouring_basic.bul wtih unchanged graph_colouring_basic.bul
output is

Instance ground. Starts solving
SAT
Model 1: ~set_a_blue ~set_a_green ~set_a_red ~set_e_blue ~set_e_green ~set_e_red

If running bule2 --solve --models 0 graph_colouring_basic.bul --solver minisat, the output is as expected

Instance ground. Starts solving
SAT
Model 1: ~set_a_blue ~set_a_green  set_a_red ~set_e_blue  set_e_green ~set_e_red
Model 2: ~set_a_blue ~set_a_green  set_a_red  set_e_blue ~set_e_green ~set_e_red
Model 3: ~set_a_blue  set_a_green ~set_a_red ~set_e_blue ~set_e_green  set_e_red
Model 4: ~set_a_blue  set_a_green ~set_a_red  set_e_blue ~set_e_green ~set_e_red
Model 5:  set_a_blue ~set_a_green ~set_a_red ~set_e_blue  set_e_green ~set_e_red
Model 6:  set_a_blue ~set_a_green ~set_a_red ~set_e_blue ~set_e_green  set_e_red
No more models. Total: 6 displayed models out of 6 models.