FIFO Tax Lot Gains

A brokerage tracks cost basis using first in, first out (FIFO) tax lots. You are given a chronological list of events; each event is [action, symbol, shares, price]:

• ["buy", symbol, shares, price] creates a new lot of shares shares of symbol, each bought at price.
• ["sell", symbol, shares, price] sells shares shares of symbol at price each.

A sell consumes shares from that symbol's lots starting with the oldest lot. If the oldest lot has fewer shares than still need to be sold, it is used up entirely and the sale continues with the next oldest lot. A lot that is only partly used keeps its remaining shares at their original price. Lots of different symbols never interact.

The realized gain of a sell is the sum, over every share it sells, of sell price - purchase price of that share's lot. It is negative when shares are sold at a loss.

Return a list with the realized gain of each sell event, in the order the sells occur. A sell never asks for more shares than are currently held for that symbol.