(1) In determining whether any, and if so what, balancing allowance or balancing charge is to be made to or on any person for any chargeable period in taxing a trade, there shall be deemed to have been made to that person, for every previous chargeable period in which the machinery or plant belonged to that person and which is a chargeable period to be taken into account for the purpose of this section, such wear and tear allowance or greater wear and tear allowance, if any, in respect of the machinery or plant as would have been made to that person if all the conditions specified in subsection (3) had been fulfilled in relation to every such previous chargeable period.
(2) There shall be taken into account for the purposes of this section every previous chargeable period in which the machinery or plant belonged to the person and -
(a) during which the machinery or plant was not used by the person for the purposes of the trade,
(b) during which the trade was not carried on by the perso
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