function - a function.

  #'(lambda (*standard-output* &rest arguments)
      (apply #'format t control-string arguments)
      arguments-tail)

where arguments-tail is either the tail of arguments which has as its car the argument that would be processed next if there were more format directives in the control-string, or else nil if no more arguments follow the most recently processed argument.

(funcall (formatter "~&~A~A") *standard-output* 'a 'b 'c)
 AB
 (C)

(format t (formatter "~&~A~A") 'a 'b 'c)
 AB
  NIL