code: function _addPreferredTerm(mapping, term, entry, typeOrLanguageValue) {
      if(!(typeOrLanguageValue in entry)) {
        entry[typeOrLanguageValue] = term;
      }
    }
docs: Adds the term for the given entry if not already added.
  
  @param mapping the term mapping.
  @param term the term to add.
  @param entry the inverse context typeOrLanguage entry to add to.
  @param typeOrLanguageValue the key in the entry to add to.