I dont think that it is impossible, if they were able to get both feet in at once, they could probably do it, but yeah, getting over the head (the simple part) quickly runs into trouble with the shoulder. There has been a few studies, done on this to see whether even the largest of snakes (retics, anaconda etc)could eat a person and they come back with the same result - good ol shoudlers. Punja, yeah you are correct in that, there is no doubt that a big python could constict the life from someone, but in terms of eating them the shoulders are the problem.
