I recommend to use :additional-nspace-list argument.
In many case, we need to specify or write float-vector directory for :null-space arguments.
However, sometimes it does not work 
if you use another link-list such as ":use-torso t" or "fullbody-inverse-kinematics",  the robot changes, ...,and so on.

How to use :additional-nspace-list:
```
:additional-nspace-list
 (list
   (list (send *robot* :larm :shoulder-r :child-link) (* sign vel))
  )
```

To summarize, 
- :weight or :null-space argument (recommended for developper)
  
  If users want to specify weight or null-space as float-vector, use :weight or :null-space.
- :additional-weight-list or :additional-nspace-list (recommended for users)
  
  If users want to specify the joint to set weight or nspace, use :additional-weight-list or :additional-nspace-list.