change(src, len = 1, _id) {
        let id = this._tsid(_id, `change(${len})`)
        return this.ts(src[0] - src[len], id, src.__tf__)
    }
Change: x[0] - x[len]
@param {TS} src - Input
@param {number} [len] - Length
@return {TS} - New time-series