code: def find(public_id_or_alias, value, recursive=false)
        pid = ControlledValue.standard_public_id(public_id_or_alias)
        value_cv_hash = @pid_value_cv_hash[pid]
        cv = value_cv_hash[value]
        if recursive then
          fetch_descendants(cv, value_cv_hash)
        end
        cv
      end
docs: Returns the CV with the given public_id_or_alias and value. Loads the CV if necessary
   from the database. The loaded CV does not have a parent or children.
  
   @param [String, Symbol] public_id_or_alias the caTissue public id or alias
   @param [String] value the CV value
   @param [Boolean] recursive whether to load the CV children as well.
   @return [CaRuby::ControlledValue, nil] the matching CV, or nil if no match