user: Question: What is the model number introduced May 1999?
Columns: ['Model', 'Introduction', 'Discontinued', 'CPU Speed', 'Print resolution (DPI) Resolution is given in dots per inch (DPI)', 'Print speed (PPM)', 'Standard memory', 'Maximum memory']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [4000.0, 'November 1997', 'May 1999', '100MHz', '1200', '17', '4 MB', '100 MB']
SQL:

assistant: SELECT "Model" FROM "Table" WHERE "Introduction" = 'May 1999';