It wasn't until the mermaid turned up in his bathtub that Simon Grue seriously began to wonder what the Russians were doing on the roof next door. The old house next door together with its spacious tarpapered roof, which held a sort of pent-shack, a cylindrical old water tank, and several chicken-wire enclosures, had always been a focus of curiosity in this region of Greenwich Village, especially to whoever happened to be renting Simon's studio, the north window-cum-skylight of which looked down