Comment by rssoconnor

I have the following file called oop.nix dated from that time.

    # Object Oriented Programming library for Nix
    # By Russell O'Connor in collaboration with David Cunningham.
    #
    # This library provides support for object oriented programming in Nix.
    # The library uses the following concepts.
    #
    # A *class* is an open recursive set.  An open recursive set is a function from
    # self to a set.  For example:
    #
    #     self : { x = 4; y = self.x + 1 }
    #
    # Technically an open recursive set is not recursive at all, however the function
    # is intended to be used to form a fixed point where self will be the resulting
    # set.
    #
    # An *object* is a value which is the fixed point of a class.  For example:
    #
    #    let class = self : { x = 4; y = self.x + 1; };
    #        object = class object; in
    #    object
    #
    # The value of this object is '{ x = 4; y = 5; }'.  The 'new' function in this
    # library takes a class and returns an object.
    #
    #     new (self : { x = 4; y = self. x + 1; });
    #
    # The 'new' function also adds an attribute called 'nixClass' that returns the
    # class that was originally used to define the object.
    #
    # The attributes of an object are sometimes called *methods*.
    #
    # Classes can be extended using the 'extend' function in this library.
    # the extend function takes a class and extension, and returns a new class.
    # An *extension* is a function from self and super to a set containing method
    # overrides.  The super argument provides access to methods prior to being
    # overloaded.  For example:
    #
    #    let class = self : { x = 4; y = self.x + 1; };
    #        subclass = extend class (self : super : { x = 5; y = super.y * self.x; });
    #    in new subclass
    #
    # denotes '{ x = 5; y = 30; nixClass = <LAMBDA>; }'.  30 equals (5 + 1) * 5).
    #
    # An extension can also omit the 'super' argument.
    #
    #    let class = self : { x = 4; y = self.x + 1; };
    #        subclass = extend class (self : { y = self.x + 5; });
    #    in new subclass
    #
    # denotes '{ x = 4; y = 9; nixClass = <LAMBDA>; }'.
    #
    # An extension can also omit both the 'self' and 'super' arguments.
    #
    #    let class = self : { x = 4; y = self.x + 1; };
    #        subclass = extend class { x = 3; };
    #    in new subclass
    #
    # denotes '{ x = 3; y = 4; nixClass = <LAMBDA>; }'.
    #
    # The 'newExtend' function is a composition of new and extend.  It takes a
    # class and and extension and returns an object which is an instance of the
    # class extended by the extension.
    
    rec {
      new = class :
        let instance = class instance // { nixClass = class; }; in instance;
    
      extend = class : extension : self :
        let super = class self; in super //
         (if builtins.isFunction extension
          then let extensionSelf = extension self; in
               if builtins.isFunction extensionSelf
               then extensionSelf super
               else extensionSelf
          else extension
         );
    
      newExtend = class : extension : new (extend class extension);
    }

In nix overlays, the names "final" and "prev" used to be called "self" and "super", owing to this OOP heritage, but people seemed to find those names confusing. Maybe you can still find old instances of the names "self" and "super" in places.