Maps represent collections with explicit keys.  
| <map> | (<col>) | C | 
| <tab> | (<map> <col!>) | C | 
|  | Tables are near constant-time aggregate data structures.  
Users can define their own tables by subclassing and overriding the
key-test and tab-hash methods. |  | 
| tab-growth-factor | (x|<tab> => <flo>) | P | 
|  | factor by which to grow capacity. |  | 
| tab-growth-threshold | (x|<tab> => <flo>) | P | 
|  | when to grow based on proportion of total table capacity. |  | 
| tab-shrink-threshold | (x|<tab> => <flo>) | P | 
|  | when to shrink based on proportion of total table capacity. |  | 
| tab-hash | (x|<tab> => <fun>) | G | 
|  | returns hash function. |  | 
| id-hash | (x|<tab> => <int>) | G | 
|  | hash function based on identity. |  | 
|  |