crossunder(src1, src2, _id) {
        let id = this._tsid(_id, `crossunder`)
        let x = (src1[0] < src2[0]) && (src1[1] >= src2[1])
        return this.ts(x, id, src1.__tf__)
    }
When one time-series goes under another one
@param {TS} src1 - TS1
@param {TS} src2 - TS2
@return {TS} - New time-series