code: func (s *SelectStatement) Reduce(valuer Valuer) *SelectStatement {
  	stmt := s.Clone()
  	stmt.Condition = Reduce(stmt.Condition, valuer)
  	for _, d := range stmt.Dimensions {
  		d.Expr = Reduce(d.Expr, valuer)
  	}
  
  	for _, source := range stmt.Sources {
  		switch source := source.(type) {
  		case *SubQuery:
  			source.Statement = source.Statement.Reduce(valuer)
  		}
  	}
  	return stmt
  }
docs: // Reduce calls the Reduce function on the different components of the
  // SelectStatement to reduce the statement.