code: public void setIndent(Integer indent) {
  		if (indent == null) {
  			remove(OutputKeys.INDENT);
  			remove(INDENT_AMT);
  			return;
  		}
  
  		if (indent < 0) {
  			throw Messages.INSTANCE.getIllegalArgumentException(11);
  		}
  
  		put(OutputKeys.INDENT, "yes");
  		put(INDENT_AMT, indent.toString());
  	}
docs: Sets the number of indent spaces to use for pretty-printing (disabled by
  default).
  @param indent the number of indent spaces to use or null to disable
  pretty-printing
  @throws IllegalArgumentException if the indent amount is less than zero