code: func (b *lowBuilder) dbUnique() string {
  	if b.high.DBUnique != "" {
  		return b.high.DBUnique
  	}
  	if b.high.Identity != "" {
  		return strings.ToLower(b.high.Identity)
  	}
  	return osutil.Username() // may be empty, if $USER unset
  }
docs: // dbUnique returns the uniqueness string that is used in databases names to
  // differentiate them from databases used by other Perkeep instances on the same
  // DBMS.