code: @Nonnull
    public final HCOutput setFor (@Nullable final IHCHasID <?> aFor)
    {
      if (aFor == null)
        m_sFor = null;
      else
        m_sFor = aFor.ensureID ().getID ();
      return this;
    }
docs: Specifies the relationship between the result of the calculation, and the
  elements used in the calculation
  
  @param aFor
  The HTML of the other object.
  @return this