explore col 1
    row 1
        place
        explore col 2
            row 1--dead end (not safe)
            row 2--dead end (not safe)
            row 3
                place
                explore col 3
                    row 1--dead end (not safe)
                    row 2--dead end (not safe)
                    row 3--dead end (not safe)
                    row 4--dead end (not safe)
                remove
            row 4
                place
                explore col 3
                    row 1--dead end (not safe)
                    row 2
                        place
                        explore col 4
                            row 1--dead end (not safe)
                            row 2--dead end (not safe)
                            row 3--dead end (not safe)
                            row 4--dead end (not safe)
                        remove
                    row 3--dead end (not safe)
                    row 4--dead end (not safe)
                remove
        remove
    row 2
        place
        explore col 2
            row 1--dead end (not safe)
            row 2--dead end (not safe)
            row 3--dead end (not safe)
            row 4
                place
                explore col 3
                    row 1
                        place
                        explore col 4
                            row 1--dead end (not safe)
                            row 2--dead end (not safe)
                            row 3
                                place
                                explore col 5
                                    col > size, so this is a solution
                                    return true
                            return true
                    return true
            return true
    return true