@_ = [3-1]
3 * 1 > 0 && up(2-0, 3-1) # decremented both row and column are passed plus original @_
    @_ = [2-0 3-1]
    2 * 0 > 0 is FALSE...
    2 > 0 && up(1-0, 2-0, 3-1) # decremented row and original column are passed plus original @_
        @_ = [1-0 2-0 3-1]
        1 * 0 > 0 is FALSE...
        1 > 0 && up(0-0, 1-0, 2-0, 3-1) # decremented row and original column are passed plus original @_
            @_ = [0-0 1-0 2-0 3-1]
            RETURNING: @_ is a valid path [0-0 1-0 2-0 3-1]
3 > 1 && up(2-1, 3-1) # decremented row and original column are passed plus original @_
    @_ = [2-1 3-1]
    2 * 1 > 0 && up(1-0, 2-1, 3-1) # decremented both row and column are passed plus original @_
        @_ = [1-0 2-1 3-1]
        1 * 0 > 0 is FALSE...
        1 > 0 && up(0-0, 1-0, 2-1, 3-1) # decremented row and original column are passed plus original @_
            @_ = [0-0 1-0 2-1 3-1]
            RETURNING: @_ is a valid path [0-0 1-0 2-1 3-1]
    2 > 1 && up(1-1, 2-1, 3-1) # decremented row and original column are passed plus original @_
        @_ = [1-1 2-1 3-1]
        1 * 1 > 0 && up(0-0, 1-1, 2-1, 3-1) # decremented both row and column are passed plus original @_
            @_ = [0-0 1-1 2-1 3-1]
            RETURNING: @_ is a valid path [0-0 1-1 2-1 3-1]
        1 > 1 is FALSE...
"0-0 1-0 2-0 3-1\n0-0 1-0 2-1 3-1\n0-0 1-1 2-1 3-1\n"