code: protected static double subtractedMagnitude(DoubleVector c, DoubleVector v) {
          return Math.sqrt(subtractedMagnitudeSqrd(c, v));
      }
docs: Returns the magnitude of {@code c} as if {@code v} was added to the the
  vector.  We do this because it would be more costly, garbage collection
  wise, to create a new vector for each alternate cluster and * vector.