SQLite

** A database connection contains a pointer to an instance of
** this object for every database file that it has open.  This structure
** is opaque to the database connection.  The database connection cannot
** see the internals of this structure and only deals with pointers to
** this structure.
**
** For some database files, the same underlying database cache might be 
** shared between multiple connections.  In that case, each contection
** has it own pointer to this object.  But each instance of this object
** points to the same BtShared object.  The database cache and the
** schema associated with the database file are all contained within
** the BtShared object.
**
** All fields in this structure are accessed under sqlite3.mutex.
** The pBt pointer itself may not be changed while there exists cursors 
** in the referenced BtShared that point back to this Btree since those

/*
** A cursor is a pointer to a particular entry within a particular
** b-tree within a database file.
**
** The entry is identified by its MemPage and the index in
** MemPage.aCell[] of the entry.
**
** When a single database file can shared by two more database connections,
** but cursors cannot be shared.  Each cursor is associated with a
** particular database connection identified BtCursor.pBtree.db.
**
** Fields in this structure are accessed under the BtShared.mutex
** found at self->pBt->mutex. 
*/
struct BtCursor {