code: function getSubField (field, index, property) {
      // get list of sub fields by index
      var fields = field.values.length > index
          ? field.values[index]
          : field.fields
      // find field for property
      return _.find(fields, field => field.property === property)
  }
docs: /* private functions 
  @function getSubField
  
  get sub field by index and property name from field
  
  @param {object} field
  @param {integer} index
  @param {string} property
  
  @returns {object}