code: func (a Sources) Measurements() []*Measurement {
  	mms := make([]*Measurement, 0, len(a))
  	for _, src := range a {
  		switch src := src.(type) {
  		case *Measurement:
  			mms = append(mms, src)
  		case *SubQuery:
  			mms = append(mms, src.Statement.Sources.Measurements()...)
  		}
  	}
  	return mms
  }
docs: // Measurements returns all measurements including ones embedded in subqueries.