A pointer to an Objective-C foreign object.
A selector of type
A string or
A pointer to an Objective-C foreign object or
A notification center.
add-observer calls the Objective-C instance method
"addObserver:selector:name:object:" of center to add target as an observer for selector with the given name and object, which both default to
LispWorks Objective-C and Cocoa Interface User Guide and Reference Manual - 15 Feb 2015