get variables() {
        const _i = this._inner;
        if (!_i.variables) {
            _i.variables = this.columns.map(c => c.variable + c.castText).join();
        }
        return _i.variables;
    }
@name helpers.ColumnSet#variables
@type string
@readonly
@description
Returns a string - formatting template for all column values.

@see {@link helpers.ColumnSet#assign assign}

@example
const cs = new ColumnSet(['id^', {name: 'cells', cast: 'int[]'}, 'doc:json']);
console.log(cs.variables);
//=> ${id^},${cells}::int[],${doc:json}