This function checks whether the input is proper index vector, i.e., a
vector of positive integer values.
     
    
    Usage
    is_index_vector(x, error = TRUE)
 
    
    Arguments
    - x
- Any object. 
- error
- In the case that - xis not a proper index vector, either- TRUE(default) to throw an error or- FALSEto return invisibly- FALSE.
 
 
    
    Value
    
If error = TRUE, either invisibly TRUE or an error is thrown.
If error = FALSE, invisibly TRUE or FALSE.