One of these walks used to take them down a narrow sidestreet in a busy part of London. It was a clean, busy, friendlystreet with bright little shops and shiny doorknockers. Nearthe end of this street, however, stood a dark, mysterious,windowless building.The door had neither bell nor knockerand looked dusty and uncared for. Dirty children played fearlessly on the doorstep, and nobody ever opened the door todrive them away.