undouble

From un- +‎ double.

undouble (third-person singular simple present undoubles, present participle undoubling, simple past and past participle undoubled)

1. (transitive) To undo the doubling of; to unfold, or render single again.
   • 2014, Alexander A. Stepanov, Daniel E. Rose, From Mathematics to Generic Programming, page 54:

     So to simulate this, the iterative version needs to repeatedly “undouble” the value, which it will do by calling a function half.