Capital gains exemption tied to reinvestment in residential property; assessments must be amended when purchase or construction occurs timely. Amendment extends the time limits in section 155 for acquiring or constructing a residential house to qualify under section 54F and inserts sub-section (10C) requiring the Income-tax Officer to amend assessments to exclude exempted capital gain where the assessee purchases within the short-term period or constructs within the longer period; section 154 procedures apply, with its four-year limitation reckoned from the assessment date.