[IAEP] New activity: Arithmetic.

[Albert Cahalan]

First of all, it's wonderful to finally see this activity.

Plenty of words in the UI are not easy, starting with "difficulty". :-)

There doesn't seem to be any scratch space to work in, but I'm just
looking at the screen shot. Can the user lay out a long division in
the standard form? Can the user have some place to write out extra
numbers for borrow/carry (optionally tiny) and possibly cross out the
original numbers? There are at least two styles for this, with tiny
numbers probably the norm when doing multi-digit multiplication.

The 3 difficulty levels are kind of vague. Just for addition I can
think of...

0..9 plus 0..9 resulting in 0..9
0..9 plus 0..9 resulting in 0..18
0..9 plus 0..9 plus optional-one resulting in 0..19
multi-digit w/o carry
multi-digit w/ carry, no change in number of digits
arbitrary multi-digit

That's w/o even considering decimals, negative numbers, fractions,
and worse. Subtraction has an extra level, because borrowing is
harder when you need to borrow from a zero.