Deepcopy interrupt the execution.

[muchang]

When taking QF_NRA benchmarks atan-vega-3-chunk-0596.smt2 and ball_count_2d_hill_simple.04.seq_lazy_global_13.smt2 as the fusion seeds, yinyang will unexpectedly terminate without giving any information. The root cause seems to be that deepcopy terminates the execution at the following line:

yinyang/yinyang/src/mutators/SemanticFusion/SemanticFusion.py

Line 145 in f38bb10

copy.deepcopy(self.formula1), copy.deepcopy(self.formula2)

We need to investigate further to see whether it is a bug in deepcopy and how to address it.