Return the assertion buffer.
The assertion buffer is used to buffer statements that are being asserted
so as to maximize the opportunity for batch writes. Truth maintenance (if
enabled) will be performed no later than the commit of the transaction.
Note: The same
#buffer is reused by each loader so that we can on
the one hand minimize heap churn and on the other hand disable auto-flush
when loading a series of small documents. However, we obtain a new buffer
each time we perform incremental truth maintenance.
Note: When non-null
and non-empty, the buffer MUST be
flushed (a) if a transaction completes (otherwise writes will not be
stored on the database); or (b) if there is a read against the database
during a transaction (otherwise reads will not see the unflushed
statements).
Note: if truthMaintenance is enabled then this buffer is backed by a
temporary store which accumulates the
SPOs to be asserted.
Otherwise it will write directly on the database each time it is flushed,
including when it overflows.