o Rename "pop" method into "put".
o rename "kill" into "remove";
o disable assertion in the explode(), turns out the digger code
  can actually calll explode several times.
Rename method "pop" into "put" for clarity.
Move some of the code that control explosions here.
pop -> put.
pop -> put.
o pop -> put;

o move some of the bullet code into bullet_obj.