You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: expr.go
    path of file: ./repos/manba/pkg/expr
    
    the code of the file until where you have to start completion: package expr

import (
	"bytes"
	"fmt"
	"strings"

	"github.com/buger/jsonparser"
	"github.com/fagongzi/util/hack"
	"github.com/valyala/fasthttp"
)

const (
	eoi    byte = 0x1A
	dollar      = byte('$')
	lParen      = byte('(')
	rParen      = byte(')')
)

const (
	tokenEOF = iota
	tokenUnknown
	tokenDollar
	tokenLParen
	tokenRParen
)

var (
	origin = []byte("origin")
	path   = []byte("path")
	query  = []byte("query")
	cookie = []byte("cookie")
	header = []byte("header")
	body   = []byte("body")
	depend = []byte("depend")
	param 
    
func newExpr(value []byte) (Expr, error) {
	values := bytes.Split(value, dot)
	if bytes.Equal(values[0], origin) {
		return newOriginExpr(values)
	} else if bytes.Equal(values[0], depend) {
		if len(values) < 2 {
			return nil, fmt.Errorf("syntax error: depend error")
		}

		return &dependParamExpr{param: toStringSlice(values[1:])}, nil
	} else if bytes.Equal(values[0], param) {
		if len(values) != 2 {
			return nil, fmt.Errorf("syntax error: param error")
		}

		return &pathParamExpr{param: hack.SliceToString(values[1])}, nil
	}

	return nil, fmt.Errorf("syntax error: not support source %s", values[0])
}