Whenever a commodity is exchanged for another in a posting, one of the
two is considered @emph{primary}, and the other secondary.
Primariness of a commodity is remembered, since the @option{--market}
option only renders balances into secondary commodities, never
primaries.  To render primaries, use the @option{--exchange=COMMODITY}
option.

In all of the following examples, the P commodity is considered primary
and the S is secondary (the P at the beginning of the line indicates a
price-setting directive):

@smallexample
2009/01/01 Sample 1a
    Assets:Brokerage:Stocks                100 S
    Assets:Brokerage:Cash                 -100 P

P 2009/01/15 00:00:00 S 2 P

2009/02/01 Sample 2a
    Assets:Brokerage:Stocks                100 S @ 1 P
    Assets:Brokerage:Cash

P 2009/02/01 00:00:00 S 4 P

2009/03/01 Sample 3a
    Assets:Brokerage:Stocks                100 S @@ 100 P
    Assets:Brokerage:Cash

P 2009/03/01 00:00:00 S 8 P

2009/04/01 Sample 4a
    Assets:Brokerage:Cash                  100 P
    Assets:Brokerage:Stocks               -100 S @{1 P@}

P 2009/04/01 00:00:00 S 16 P
@end smallexample