  For a functionally generated portfolio, there is a natural decomposition of
the relative log-return into the log-change in the generating function and a
drift process. In this note, this decomposition is extended to arbitrary stock
portfolios by an application of Fisk-Stratonovich integration. With the
extended methodology, the generating function is represented by a structural
process, and the drift process is subsumed into a trading process that measures
the profit and loss to the portfolio from trading.
