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Access the specified element, with bounds checking.
value& at( string_view key);
            Returns a reference to the mapped value of the element that matches
            key, otherwise throws.
          
            Constant on average, worst case linear in size().
          
Strong guarantee.
A reference to the mapped value.
| Name | Description | 
|---|---|
| 
                       | The key of the element to find. | 
| Type | Thrown On | 
|---|---|
| 
                       | if no such element exists. |