At one stage in Game7 in Lemmings Jr. the way I first solved it, the situation can be described as follows. See Figure .
All but one of the living lemmings are in two groups. Some of the lemmings in each group may be still pouring out of a lemming source, but once out the lemmings in each group are bounded by a pair of blockers. These lemmings will go back and forth between the blockers until the player does something about it. In this description nothing is said about the individual lemmings in the two groups. However, when we later give a special role to a lemming in each group, the aggregation changes.
The remaining lemming has parachuted to the lower level and is building the bridges needed for all the lemmings to reach the goal. When this lemming has built the bridges, it will vanish into the goal. Then the player starts a random lemming in each group digging. Eventually each group will fall through to the lower level and march back and forth between barriers. One of these lemmings will be set to building a bridge over the leftmost barrier. When this is done all the remaining lemmings will pass over the barrier and over the bridges built earlier and on to the goal.
The beginning of this game requires more consideration of parallel processes. The first lemming out of the left source is made into a blocker to prevent other lemmings from falling into some water. The second lemming is made into a parachutist and will jump down to build bridges. The third lemming is made into a blocker immediately after it passes the left lemming source after being reflected by the blocker. While all this is being completed, lemmings are coming out of the rightmost source and marching to the right. As soon as the player finishes the tasks on the left he makes the lead lemming on the right into a blocker so as to keep the lemmings marching to the right from reaching something that will kill them. After the rightmost blocker has been installed a left hand blocker for the right hand group is put in place to the left of the right hand lemming source. There is now plenty of time for bridge building.
In the meantime the parachutist is safely marching back and forth between two barriers. He needs to build a bridge over the left barrier, but he cannot reach high enough on this barrier if he starts toward it. Instead he must build towards the right barrier starting from a point that will end a bridge at the right height. Too high and he will go over the right barrier, which isn't wanted. At the right height, when he reverses he can be made to build a bridge toward the left barrier starting at the top of the bridge he has just built. This will get him over the left barrier. After that he must build a bridge over a lemming trap and another bridge up to the slope leading to the goal.
Finally, a lemming in each group is made into a digger. As the hole it digs gets deeper all the lemmings fall into it. Finally it breaks through the ceiling of the lower level, and all the lemmings fall down and can march to goal over the bridges that have been built.
One more tactical fact. All the remaining lemmings from the two sources must be allowed to fall into the holes being dug before the diggers are allowed to break through the ceiling of the lower region. Otherwise, some lemmings will fall through the hole and down to the lower level in one fall, and this will kill them.
The four lemmings used as blockers cannot be saved, and it is convenient to blow them up when the rest of the work has been done.
The above description is accurate for the way I did it then and indicates the kinds of formalizations that my old method required. There are better ways of solving that game, and they require formalizations of different facts about the lemming world and different facts about the specific situations.