move inner method into internal package

[JacksonTian]

You need to complete

• unit tests and/or feature tests
• documentation is changed or added

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.84%. Comparing base (f0e98e4) to head (f104880).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
- Coverage   99.84%   99.84%   -0.01%     
==========================================
  Files          20       21       +1     
  Lines        1286     1280       -6     
==========================================
- Hits         1284     1278       -6     
  Partials        2        2              

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