conversion, demolition, and other causes.  As a result, more than two thousand or over ten percent of the controlled rental units in the city have been removed from the housing market since 1970, and the vacancy rate has fallen below one percent.  In order to carry out the purposes of the Act, and to continue to provide a sufficient supply of decent, affordable rental housing accommodations especially for families of low and moderate income and for elderly people on fixed incomes, it is necessary ... to regulate the removal of controlled rental units from the market.


6
Cambridge City Code Sec. 8.44.010.


7
The Ordinance applies to all housing units offered for rent prior to August 13, 1979.  It prohibits an owner from "removing" any such unit from theunits to cooperative ownership without first obtaining removal permits.


13
In November 1980, the Southview plaintiffs again applied for permits, this time seeking to withdraw forty-seven rental units from the housing market.  After a hearing, the Board again demurred.  The plaintiffs did not pursue judicial review of the Board's action.


14
2. The Blevins Plaintiffs.    This group comprises two trusts which together own four apartments buildings in Cambridge, aggregating more than 170 rental units.4   In 1981, Charles Blevins formed a pair of condominium associations, intending to convert two of the properties into condominium aggregates and market the units.  The Ordinance required that Blevins obtain permits to implement this plan.  He never tried to do so--the trustees allege that such an attempt would have