RPUSHX
Syntax
        RPUSHX key element [element ...]
- Available since:
 - 2.2.0
 - Time complexity:
 - O(1) for each element added, so O(N) to add N elements when the command is called with multiple arguments.
 - ACL categories:
 - 
              
                
@write,@list,@fast, 
Inserts specified values at the tail of the list stored at key, only if key
already exists and holds a list.
In contrary to RPUSH, no operation will be performed when key does not yet
exist.
Examples
RESP2/RESP3 Reply
Integer reply: the length of the list after the push operation.History
- Starting with Redis version 4.0.0: Accepts multiple 
elementarguments.