func null() value {
	return value{}
}
Create a new null value
package interp
import (
	"fmt"
	"math"
	"strconv"
	"strings"
)
type valueType uint8
const (
	typeNull valueType = iota
	typeStr
	typeNum
	typeNumStr
)
type value struct {
	typ valueType  
	s   string     
	n   float64    
}