code: public Group<E> group(String name) {
          for (Group<E> group : this.groups()) {
              if (group.expr instanceof Expression.NamedGroup<?>) {
                  Expression.NamedGroup<E> namedGroup = (Expression.NamedGroup<E>) group.expr;
                  if (namedGroup.name.equals(name)) {
                      return group;
                  }
              }
          }
  
          return null;
      }
docs: Retrieve a group by name.
  @param name the name of the group to retrieve.
  @return the associated group.