cmd_unmount

command: /unmount

quick ref:

/unmount [@entityname]

description:

unmount player or the given entity from its currently riding entity. 
@param entityname: name of the entity, if nil, it means the calling entity, such as inside the entity's inventory.  
e.g.
/unmount      : unmount the player or calling entity
/unmount @a   : unmount closest player entity