deferred class
	DURATION

General
	cluster: time
	description: "Temporal intervals"

Ancestors
	GROUP_ELEMENT
	PART_COMPARABLE

Queries
	infix ">=" (other: [like Current] DURATION): BOOLEAN
	infix ">" (other: [like Current] DURATION): BOOLEAN
	infix "<=" (other: [like Current] DURATION): BOOLEAN
	infix "<" (other: [like Current] DURATION): BOOLEAN
	infix "-" (other: [like Current] DURATION): [like Current] DURATION
	infix "+" (other: [like Current] DURATION): [like Current] DURATION
	prefix "-": [like Current] DURATION
	prefix "+": [like Current] DURATION
	is_negative: BOOLEAN
	is_positive: BOOLEAN
	is_zero: BOOLEAN
	zero: [like Current] DURATION

Constraints
	sign correctness