func FIELD(name string) Expr {
	return &field{name}
}
FIELD creates an Expr that obtains its value from a named field.
package expr
import (
	"time"
	"github.com/getlantern/goexpr"
)
func IsField(e Expr) (string, bool) {
	f, ok := e.(*field)
	if !ok {
		return "", false
	}
	return f.Name, true
}