code: function validString(method, elem, i, j, k) {
          if ('string' !== typeof elem || elem.trim() === '') {
              throw new Error('Roler.' + method + ': roles map (' + i + ',' + j +
                              ',' + k +') has invalid elements: ' + elem);
          }
      }
docs: ### validString
  
  Validates the content of role/id or id/role match, throws errors
  
  @param {string} method The name of the method invoking validation
  @param {mixed} elem The element to validate (should
  be non-empty string: id or role)
  @param {number} i The i-th round in the matches array
  @param {number} j The j-th match at round i-th in the matches array
  @param {string} k The name of the key containig mapping to elem
  
  @see validString