click or drag from one atom to another in the same row/column with nothing between them.
each click adds a line. 1, then 2, then 3, then it clears.
lines can't cross. numbers on top say how many are left.
done = all zeros and everything joined up.
click the middle of a line to take one back. u = undo, r = clear.
after 11 it just keeps making them up, harder each time.