S. 11 : Property held for charitable purposes-Educational trust-Trust not registered under section 12AA at time of filing return-Registration obtained subsequently after assessment-Rectification application under section 154 rejected-Assessee cannot be denied legitimate deductions otherwise available as regular assessee-Purpose of assessment to recover legitimate tax and not unjust tax Assessing Officer not expected to act mechanically to fasten unjust tax liability-Matter remitted to Assessing Officer for fresh assessment order. [S.12, 12A, 12AA, 139, 143(1), 154]