code: func (q *SearchQuery) addContinueConstraint() error {
  	cont := q.Continue
  	if cont == "" {
  		return nil
  	}
  	if q.Constraint.onlyMatchesPermanode() {
  		tokent, lastbr, ok := parsePermanodeContinueToken(cont)
  		if !ok {
  			return errors.New("Unexpected continue token")
  		}
  		if q.Sort == LastModifiedDesc || q.Sort == CreatedDesc {
  			var lastMod, lastCreated time.Time
  			switch q.Sort {
  			case LastModifiedDesc:
  				lastMod = tokent
  			case CreatedDesc:
  				lastCreated = tokent
  			}
  			baseConstraint := q.Constraint
  			q.Constraint = &Constraint{
  				Logical: &LogicalConstraint{
  					Op: "and",
  					A: &Constraint{
  						Permanode: &PermanodeConstraint{
  							Continue: &PermanodeContinueConstraint{
  								LastCreated: lastCreated,
  								LastMod:     lastMod,
  								Last:        lastbr,
  							},
  						},
  					},
  					B: baseConstraint,
  				},
  			}
  		}
  		return nil
  	}
  	return errors.New("token not valid for query type")
  }
docs: // addContinueConstraint conditionally modifies q.Constraint to scroll
  // past the results as indicated by q.Continue.