Fix bug which positions access did not trigger updating

[zero-element]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 45.50%. Comparing base (dadbc4d) to head (aeb0f1c).
Report is 2 commits behind head on master.

Files	Patch %	Lines
bt/core.py	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #438   +/-   ##
=======================================
  Coverage   45.50%   45.50%           
=======================================
  Files           4        4           
  Lines        1936     1936           
  Branches      449      449           
=======================================
  Hits          881      881           
  Misses        998      998           
  Partials       57       57           

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