define :queue [
    init: method [][
        this\items: []
    ]

    empty?: method [][
        zero? this\items
    ]

    push: method [item][
        this\items: this\items ++ item
    ]

    pop: method [][
        ensure -> [not? this\empty?]

        result: this\items\0
        this\items: remove.index this\items 0
        return result
    ]
]