Refactorring skeleton of SetCriterion

[zhiqwang]

Fixing #60

[codecov]

Codecov Report

Merging #125 (1c9431e) into master (82d6afb) will increase coverage by 0.77%.
The diff coverage is 96.42%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
+ Coverage   83.56%   84.34%   +0.77%     
==========================================
  Files           8       11       +3     
  Lines         779      811      +32     
==========================================
+ Hits          651      684      +33     
+ Misses        128      127       -1     

Flag	Coverage Δ
unittests	84.34% <96.42%> (+0.77%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
test/test_engine.py	94.32% <93.65%> (-5.68%)	⬇️
test/test_hooks_utils.py	100.00% <100.00%> (ø)
test/test_models.py	100.00% <100.00%> (+5.02%)	⬆️
test/test_models_anchor_utils.py	100.00% <100.00%> (ø)
test/test_models_utils.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 82d6afb...1c9431e. Read the comment docs.

[zhiqwang]

Let's merge this PR firstly, and work on a new stream.