code: func (f *FilterOp) Range(field string, gte interface{},
  	gt interface{}, lte interface{}, lt interface{}, timeZone string) *FilterOp {
  
  	if f.RangeMap == nil {
  		f.RangeMap = make(map[string]RangeFilter)
  	}
  
  	f.RangeMap[field] = RangeFilter{
  		Gte:      gte,
  		Gt:       gt,
  		Lte:      lte,
  		Lt:       lt,
  		TimeZone: timeZone}
  
  	return f
  }
docs: // Range adds a range filter for the given field.
  // See the RangeFilter struct documentation for information about the parameters.