An assistant district attorney in Manhattan once told me, "An extraordinary thing happened yesterday in one of the criminal court parts. The judge rewrote the Constitution, just like that." I asked him for details.

"No," the prosecutor said, "you weren't there. Nobody from the press was there because we weren't arraigning a career rapist or some other celebrity. Weeks go by and no reporter ever steps into one of those courtrooms. So why should I do your job for you?"

One reporter in New York who does keep up with the lower courts, much as Charles Dickens would if he were around, is Hal Davis of The New York Post. It's from him I learned of a recent case in Family Court in which the judge did not rewrite the Constitution but rather instructed the police department that the Fourth Amendment is still on the books regardless of its hindrance to the making of speedy arrests in a program of which the police are very proud. The decision was a shock to police and prosecutors, especially coming from so far below.

At issue is the work of the taxi and robbery squad, attached to the street crime unit. The squad's mission is to deter robbery, homicide and other crimes directed against the drivers of taxicabs, both medallion and livery vehicles. These cops cruise in unmarked cars, often cabs, through neighborhoods in which there has been a high incidence of taxi drivers' being attacked.

The members of the squad look for cabs in which certain signs point to the possibility that the driver may end his shift prematurely. Among the criteria by which these plainclothes cops judge whether a cab warrants their close attention are: the time of day, the area of the city, the number of occupants of the cab, the way they're behaving, the demeanor of the driver and the vehicle's speed.

A star team in the taxi and robbery squad is that of Lt. Thomas Cafferty and police officer Harold Kukk. In the more than three years this police department strategy has been operating, Cafferty's team has made more than 400 arrests involving gun possession, most of them in taxis that looked suspicious. It is also true that several hundred stops of taxis made by Cafferty's team did not result in arrest. But nobody bats 1.000 in any line of work. On the night of Nov. 6, 1984, Cafferty and his partner, both in plainclothes, were driving north in Washington Heights, a neighborhood with a large Hispanic population. They came abreast of a livery cab heading south. Three Hispanic-looking males were in the back seat, one of them being 15-year-old Max E., who was sitting directly behind the driver. As for the driver, he, also Hispanic-looking, seemed to be talking to himself. Cafferty reported later that the driver appeared to be "aggravated or disgusted or distraught about something."

The unmarked police car made a U-turn and followed the cab as the passengers in the cab looked back. After eight blocks, the siren and the high beams of the police vehicle were turned on, and at the same time the cop saw a pumping up-and-down movement of the left shoulder of the man in the middle of the back seat. When the car stopped, the police team pulled out the passengers. Cafferty saw the butt and trigger guard of a gun protruding from the back seat. He removed an automatic pistol with three live rounds in the magazine. All three passengers were placed under arrest.

The 15-year-old Max E. wound up in Family Court, defended by Steve Hiltz of the Legal Aid Society. Judge Mortimer Getzels ruled that the cops had abused the Fourth Amendment when they stopped the cab and seized the passengers. Cafferty and his partner had no probable cause to move in; accordingly, the gun is tainted evidence.

"Blunty put," the judge said, "the rationale for the stop was that Hispanic-looking passengers riding in a livery cab in Washington Heights who did not sit motionless, eyes front, are suspicious characters where the cabbie is making faces and talking to himself." The police must be reminded regularly that "gut reaction or hunch cannot be equated with reasonable suspicion." Case dismissed.

Edward Bennett Williams, Tom Puccio and Howard Weitzman are not likely to be found in any of the family courts of this land, so it's up to the Steve Hiltzes to demonstrate occasionally to their clients that the Constitution is not an entirely alien document in their neighborhoods. Or, as Mr. Jefferson put it, "That government is the strongest of which every man feels himself a part."

What about the "distraught" cabdriver? Had he felt in danger from his passengers? The prosecutor never called him to testify, which indicates that the prosecutor may know the answer to that question.