mfi(src, len, _id) {
        let id = this._tsid(_id, `mfi(${len})`)
        let vol = this.env.shared.vol
        let ch = this.change(src, 1, id+'1')[0]

        let ts1 = this.mult(vol, ch <= 0.0 ? 0.0 : src[0], id+'2')
        let ts2 = this.mult(vol, ch  >= 0.0 ? 0.0 : src[0], id+'3')

        let upper = this.sum(ts1, len, id+'4')
        let lower = this.sum(ts2, len, id+'5')

        let res = undefined
        if (!this.na(lower)) {
            res = this.rsi(upper, lower, id+'6')[0]
        }
        return this.ts(res, id, src.__tf__)
    }
Money Flow Index
@param {TS} src - Input
@param {number} len - Length
@return {TS} - New time-series