The value of source on another symbol of the universe, on the current bar.

symbol must be a member of the strategy's universe; one outside it never has a value, so such a strategy is refused when it is compiled. source is evaluated on that symbol's own data, not this one's; it cannot carry a symbol and cannot contain another cross-sectional operand. Requires a universe or portfolio strategy with two or more symbols.

Example, QQQ's close, to compare this symbol's close with: {"kind": "sibling", "symbol": "QQQ", "source": {"kind": "price", "field": "close"}}