Extend to_power_type only if it is defined

[yuyichao]

Fix #373

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.72%. Comparing base (5ab3edd) to head (d5b00d3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #374   +/-   ##
=======================================
  Coverage   95.72%   95.72%           
=======================================
  Files          17       17           
  Lines        3207     3207           
=======================================
  Hits         3070     3070           
  Misses        137      137           

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