code: protected Float getExpressionResolution(MediaExpression e)
      {
          if (e.size() == 1) //the length requires exactly one value
          {
              Term<?> term = e.get(0);
              if (term instanceof TermResolution)
                  return dpiResolution((TermResolution) term);
              else
                  return null;
          }
          else
              return null;
      }
docs: Obtains the resolution specified by the given media query expression.
  @param e The media query expression specifying a resolution.
  @return The length converted to pixels or {@code null} when the value cannot be converted to resolution.