Normal sudoku rules apply. Each gray thermosquare is the start of a non-branching thermosnake that moves orthogonally through the grid. A thermosnake cannot touch itself orthogonally or diagonally, but it may touch any other thermosnake any number of times. Two different thermosnakes cannot enter the same cell. Each thermosnake is exactly 7 cells long, and the digits along a thermosnake increase from start to end.