overall interlock: for any two open squares, there is a path of open squares that connects the first to the second. (That is, we can get from here to there through words.) Your answer should formally...

overall interlock: for any two open squares, there is a path of open squares that connects the first to the second. (That is, we can get from here to there through words.) Your answer should formally define a predicate P(i, j, x, y) that is true exactly when there exists is a path from hi, ji to hx, yi: “there exists a sequence of open squares starting with hi, ji such that . . .”.)