docs: function handleAlias(that, nameAndAlias, method) {
          var tokens, id, alias;
          tokens = extractAlias(nameAndAlias);
          id = tokens.id;
          alias = tokens.alias;
          if (id === alias) {
              throw new Error('Stager.' + method + ': id equal to alias: ' +
                              nameAndAlias);
          }
          if (alias && !that.stages[id]) {
              throw new Error('Stager.' + method + ': alias is referencing ' +
                              'non-existing stage: ' + id);
          }
          if (alias && that.stages[alias]) {
              throw new Error('Stager.' + method + ': alias is not unique: ' +
                              alias);
          }
          return tokens;
      }