/**

* inverse

* returns the inverse of this symbol table.

* the inverse is a symbol table where the roles of the Keys and Values are switched

* Example:

* Consider the symbol table below, written as a collection of key value pairs

* A, 1

* B, 2

* C, 3

* The inverse this would be:

* 1, A

* 2, B

* 3, C

* In the original table, keys were Strings, values were Integers

* So in the inverse table, keys would be Integers and values would be Strings

* Note that the return type for the function is already correctly specified.

* That is: the invoking instance will be a Symbol table with key type: Key and value type: Value

* the inverse will be a symbol table with key type: Value and value type: Key *

* Note: if the symbol table contains duplicate values,

* you may use any of the corresponding keys for the inverse

* Example:

* A, 1

* B, 2

* C, 3

* D, 2

* 2 will become a Key in the inverse symbol table and its value could be either B or D

*/