+
+ (multiple-value-bind (focal-length o-od o-id o-m o-bf)
+ (close-up :focal-length focal-length :object-distance original-object-distance
+ :image-distance original-image-distance :magnification original-magnification :units units)
+
+ (cond
+ (extension-length
+ (multiple-value-bind (focal-length n-od n-id n-m n-bf)
+ (close-up :focal-length focal-length :image-distance (+ o-id extension-length) :units units)
+ (values o-od o-id o-m o-bf n-od n-id n-m n-bf extension-length)))
+ ((not extension-length)
+ (multiple-value-bind (focal-length n-od n-id n-m n-bf)
+ (close-up :focal-length focal-length :object-distance new-object-distance
+ :image-distance new-image-distance :magnification new-magnification :units units)
+ (values o-od o-id o-m o-bf n-od n-id n-m n-bf (- n-id o-id)))))))
+