colGetEnabledSwapGate(col) {
        if (col < 0 || col >= this.columns.length) {
            return undefined;
        }
        let locs = [];
        for (let row = 0; row < this.numWires; row++) {
            if (this.gateInSlot(col, row) === Gates.Special.SwapHalf) {
                if (this.gateAtLocIsDisabledReason(col, row) !== undefined) {
                    return undefined;
                }
                locs.push(row);
            }
        }
        if (locs.length !== 2) {
            return undefined;
        }
        return locs;
    }
@param {int} col
@returns {undefined|![!int, !int]}