rsi(x, y, _id) {
        // Check if y is a timeseries
        if (!this.na(y) && y.__id__) {
            var id = this._tsid(_id, `rsi(x,y)`)
            var rsi = 100 - 100 / (1 + this.div(x, y, id)[0])
        } else {
            var id = this._tsid(_id, `rsi(${y})`)
            let ch = this.change(x, 1, _id)[0]
            let pc = this.ts(Math.max(ch, 0), id+'1', x.__tf__)
            let nc = this.ts(-Math.min(ch, 0), id+'2', x.__tf__)
            let up = this.rma(pc, y, id+'3')[0]
            let down = this.rma(nc, y, id+'4')[0]
            var rsi = down === 0 ? 100 : (
                up === 0 ? 0 : (100 - (100 / (1 + up / down)))
            )
        }
        return this.ts(rsi, id+'5', x.__tf__)
    }
Relative Strength Index
@param {TS} x - First Input
@param {number|TS} y - Second Input
@return {TS} - New time-series