    deferred class
    	DURATION

    General
    	cluster: time
    	description: "Temporal intervals"

    Ancestors
    	GROUP_ELEMENT*
    	PART_COMPARABLE*

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

    Constraints
    	sign correctness