Collection of Numbers in Scala

We have a collection of integers which is initially empty and events of the following types:

    1. insert a new integer into the collection
    2. remove an integer from the collection
    3. return the number of occurences of a given integer

Implement all the operations above.

Example:

Input:
    insert(5) 
    insert(2)
    insert(5)
    count(5)
    remove(5)
    count(5)
Output:2 1
Explanation There are two 5s at the first count and one after the second