The loop must "enter" each cell it passes through from the center of one of its four sides and "exit" from a different side; all turns are therefore 90 degrees. The two varieties of circle have differing requirements for how the loop must pass through them: White circles must be traveled straight through, but the loop must turn in the previous and/or next cell in its path; Black circles must be turned upon, but the loop must travel straight through the next and previous cells in its path.
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