DECRBY
Syntax
        DECRBY key decrement
- Available since:
 - 1.0.0
 - Time complexity:
 - O(1)
 - ACL categories:
 - 
              
                
@write,@string,@fast, 
Decrements the number stored at key by decrement.
If the key does not exist, it is set to 0 before performing the operation.
An error is returned if the key contains a value of the wrong type or contains a
string that can not be represented as integer.
This operation is limited to 64 bit signed integers.
See INCR for extra information on increment/decrement operations.