deferred class
	GROUP_ELEMENT

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

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