# Box primitive `value` to `mtype`.
	private fun box(value: RuntimeVariable, mtype: MType): RuntimeVariable do
		if value.is_boxed then return value
		var obj_type = compiler.mainmodule.object_type
		if value.mtype isa MNullType then
			return new_expr("new RTVal(null, null)", compiler.mainmodule.model.null_type)
		end
		var mbox = value.mtype.as(MClassType).mclass
		return new_expr("new RTVal({mbox.rt_name}.get{mbox.rt_name}(), {value})", obj_type)
	end