Prevent assignment to an element in a deflist object
     
    
    Usage
    # S3 method for deflist
[[(x, i) <- value
 
    
    Arguments
    - x
- A deflist object. 
- i
- Index or name of the element to be assigned. 
- value
- Value to be assigned to the element. 
 
    
    Value
    
this function throws an error be design, no return value