反转字符串，但其指定的字串不反转

import stacks
proc revstr(src,token:string):string =
    var 
        s  = Stack[char]()
        i = 0
        sl = src.len
        tl = token.len
    while i < sl :
        if i <= sl - tl:
            if src[i..<(i+tl)] == token:
                for ti in countdown(tl - 1,0 ):
                    s.push(token[ti])
                i += tl
            else:
                s.push(src[i])
                i.inc
        else:
            s.push(src[i])
            i.inc
    while not s.empty:
        result.add(s.pop)
    return result

const
    src = "Welcome you, my friend you"
    token = "you"
echo revstr(src,token)