When an event arrives at a Flex operator from an input stream,
the method for that input stream is run.
The SPLASH code for that method has two implicitly declared variables
for each input stream:
one for the event and one for the old version of the event.
More precisely,
if the input stream is named InputStream,
the variables are:
InputStream,
with the type of record events from the input stream,
and
InputStream_old,
with the type of record events from the input stream.
When the method for input stream is run,
the variable
InputStream is bound to the event that arrived
from that stream.
If the event is an update,
the variable
InputStream_old is bound to the previous contents
of the record,
otherwise it is null.
Note: Delete events always come populated with the data previously held in the input stream.
A Flex operator can have more than one input stream.
For instance,
if there is another input stream called
AnotherInput,
the variables
AnotherInput and AnotherInput_old
are implicitly declared in the method block for InputStream.
They are set to null when the method block begins,
but can be assigned within the block.