This puzzle appears in "Challenge Your Brain Math & Logic Puzzles" (Dave Tuller & Michael Rios, Puzzle Wright Press). It is similar to Masyu.
Puzzle and Goal
An unsolved puzzle consists of a rectangular grid; some cells contain a white or black circle.
The goal is to create a single closed loop that satisfies the clues.
The solved grid must satisfy the following conditions:
The path must turn 90° in each white circle.
The path may not turn in any black circle.
The path must visit every cell.