code: public <T extends Widget & Checkable> boolean uncheck(T checkableWidget) {
          if (hasChild(checkableWidget)) {
              return checkInternal(checkableWidget, false);
          }
          return false;
      }
docs: Set the specified {@link Checkable} {@link Widget} as unchecked, if it is a child of this
  {@link CheckableGroup} and not already unchecked.
  @param checkableWidget The {@code Checkable Widget} to
  {@linkplain Checkable#setChecked(boolean) set unchecked}.
  @return {@code True} if {@code checkableWidget} is a child of this {@code CheckableGroup} and
  was not already unchecked; {@code false} otherwise.