code: protected String getExpressionIdentifier(MediaExpression e)
      {
          if (e.size() == 1) //the length requires exactly one value
          {
              Term<?> term = e.get(0);
              if (term instanceof TermIdent)
                  return ((TermIdent) term).getValue().trim().toLowerCase(Locale.ENGLISH);
              else
                  return null;
          }
          else
              return null;
      }
docs: Obtains the identifier specified by the given media query expression.
  @param e The media query expression specifying an identifier.
  @return The identifier name or {@code null} when the value cannot be converted to an identifier.