code: String getPattern(
          TextWidth width,
          PluralCategory category,
          IsoUnit unit
      ) {
  
          checkNull(width, category);
          return this.patterns.get(unit).get(width).get(category);
  
      }
docs: <p>Yields a unit pattern which optionally contains a placeholder
  of the form &quot;{0}&quot; standing for the count of units. </p>
  
  @param   width       text width (ABBREVIATED as synonym for SHORT)
  @param   category    plural category
  @param   unit        associated iso unit
  @return  unit pattern