code: function requireValidIndexes (indexes) {
      // throw error on invalid indexes option
      assert.ok(Array.isArray(indexes), 'indexes must be array')
      // validate each index spec
      _.each(indexes, index => {
          // require columns to be array
          assert.ok(Array.isArray(index.columns), 'index columns must be array')
          // require multiple columns
          assert.ok(index.columns.length > 1, 'index must have at least 2 columns')
      })
      // return validated indexes
      return indexes
  }
docs: @function requireValidIndexes
  
  validate indexes argument
  
  @param {object} database
  
  @returns {object}
  
  @throws {Error}