[dsp1234]

Can't set a bit with more than 1 adjacent (north/south/east/west) bit set.

Then you couldn't make a T (or + intersection) because as the newly incoming line was about to attach to the existing wall (regardless of any other connectivity), the final "bit" that would attach to the line and existing wall would have 2 adjacent bits set. ex: assuming a wall that travels east/west, and a line coming from the south northwards, when drawing the last pixel of the line before it attaches to the wall, the test would fail. The "south" bit would be set because that bit was set just one step prior, and the "north" bit would be set because that's where the wall is.

  ---------
      x   
      |
      |

Can't set "x" because there is more than one "adjacent" bit set (north and south).