tsi(src, short, long, _id) {
        let id = this._tsid(_id, `tsi(${short},${long})`)
        let m = this.change(src, 1, id+'0')
        let m_abs = this.ts(Math.abs(m[0]), id+'1', src.__tf__)
        let tsi = (
            this.ema(this.ema(m, long, id+'1'), short, id+'2')[0] /
            this.ema(this.ema(m_abs, long, id+'3'), short, id+'4')[0]
        )
        return this.ts(tsi, id, src.__tf__)
    }
True strength index
@param {TS} src - Input
@param {number} short - Short length
@param {number} long - Long length
@return {TS} - New time-series