thodg/cl-unix-cybernetics

diff --git a/core/defs.lisp b/core/defs.lisp
index 9383b03..9aaa643 100644
--- a/core/defs.lisp
+++ b/core/defs.lisp
@@ -55,6 +55,7 @@
 	       :initform (error "Operation without properties.")
 	       :reader operation-properties)
    (after :initarg :after
+          :initform nil
           :reader operations-before)))
 
 (defgeneric operation-generic-function (op))