use `oneunit(T)` in `construct_seeds`

[longemen3000]

closes #651 rebase of #659

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :warning: Please upload report for BASE (master@382363a). Learn more about missing BASE report. :warning: Report is 23 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #750   +/-   ##
=========================================
  Coverage          ?   87.08%           
=========================================
  Files             ?       10           
  Lines             ?      960           
  Branches          ?        0           
=========================================
  Hits              ?      836           
  Misses            ?      124           
  Partials          ?        0           

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