code: func (h *Handler) GetOfferingUsage(password, offeringID string) (*uint, error) {
  	logger := h.logger.Add("method", "GetOfferingUsage",
  		"offeringID", offeringID)
  
  	return h.uintFromQuery(logger, password,
  		`SELECT SUM(sessions.units_used)
  		   FROM channels
  		   	JOIN sessions
  			ON sessions.channel=channels.id
  			   AND channels.offering=$1`,
  		offeringID)
  }
docs: // GetOfferingUsage returns total units used for all channels
  // with a given offering.