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   to make do with 3 buffers (0, 1 & 2), but somewhere I had to add another
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   to make do with 3 buffers (0, 1 & 2), but somewhere I had to add another
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   buffer (w/c makes it 4). In most cases, the buffers "below" a number are
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   buffer (w/c makes it 4). In most cases, the buffers "below" a number are
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   only those buffers up to 2 (e.g., USE_SDF1 uses only buffers 1 & 2) so
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   only those buffers up to 2 (e.g., USE_SDF1 uses only buffers 1 & 2) so
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   3 can be used by the calling function.
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   3 can be used by the calling function.
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B. USE_SDF1, UNUSE_SDF2
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B. MAX_ATTACHED, USE_SDF1, UNUSE_SDF2
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1. The number of DB attached to the SQLite engine is limited by the value
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   of the macro MAX_ATTACHED in SQLite.
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2. By default, MAX_ATTACHED is 10. This package modifies it to 31. This
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   is the maximum, according to the SQLite docs. SQLite uses bitfields
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   somewhere, where the nth bit probably represents the nth attached db.
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3. SQLiteDF produces a lot of SDF. Most operations creates SDF where
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   intermediate values are stored.
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4. On startup, it opens workspace.db in the current directory. Then it
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   attaches SDF's listed in workspace.db.
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5. The package "juggles" with db's attached by using USE_SDF1 and UNUSE_SDF2.
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6. USE_SDF1 checks if a SDF is loaded. If not, and there is a free "slot,"
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   it is attached. If there is no free slot, then the least used among those
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   loaded is detached to make room for this one. The use count of the SDF
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   loaded is increased.
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7. On USE_SDF1, there is an option to "protect" it from being unloaded. There
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   are cases where a function uses 2 SDF and the second USE_SDF may unload
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   the 1st. In general, top-level functions that will use 2 SDF's, including
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   those that will store results in an SDF, will have to set the protect
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   statement. The protect options sets a bit in the workspace db to flag that
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1. Because of the limited
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   it should not be detached.
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8. When the protect option is used, make sure to do UNUSE_SDF2. This clears
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   the flag in workspace.db.
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9. _prepare_attach2() contains the logic for detaching SDF's.
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C. transactions
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1. when doing bulk modifications (insert/update/delete), enclose them in a
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   transaction, otherwise it will be real slow (see SQLite benchmarks in
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   SQLite site).
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2. Statements that are sqlite_step()-ed inside a transaction, even if they are
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   sqlite_prepare()-ed before a BEGIN.
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3. You may use macros _sqlite_begin and _sqlite_commit. When __SQLITE_DEBUG__
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   is defined, file and lines where the BEGIN or COMMIT occurred. Besides that
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   it is convenient.
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