    deferred class
    	GROUP_ELEMENT

    General
    	cluster: time
    	description:
    		"Invertible object with an internal + operation.

    		Note: The model is that of a commutative group."

    Ancestors
    	ANY

    Queries
    	infix "+" (other: [like Current] GROUP_ELEMENT): [like Current] GROUP_ELEMENT
    	infix "-" (other: [like Current] GROUP_ELEMENT): [like Current] GROUP_ELEMENT
    	prefix "+": [like Current] GROUP_ELEMENT
    	prefix "-": [like Current] GROUP_ELEMENT
    	zero: [like Current] GROUP_ELEMENT

    Constraints
    	neutral addition
    	self subtraction