Add optional references

[vjik]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	❌
Fixed issues	-

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@3fcebf6). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head a07f87d differs from pull request most recent head 0d9b141. Consider uploading reports for the commit 0d9b141 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #43   +/-   ##
=========================================
  Coverage          ?   99.14%           
  Complexity        ?      213           
=========================================
  Files             ?       14           
  Lines             ?      469           
  Branches          ?        0           
=========================================
  Hits              ?      465           
  Misses            ?        4           
  Partials          ?        0           

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