code: @Override
      public void undo() {
          if (!getEditor().isReadOnly() && deleted != null && Character.isLetter(deleted)) {
              getEditor().move(getBeforeLine(), getBeforeColumn());
              getEditor().delete();
              getEditor().put(deleted.toString());
              getEditor().moveLeft(1);
          }
      }
docs: Undo the {@link org.jledit.command.Command}.