Questions about reverse-engineer.

checkio([[2, 2, [4, 1]]]) # ["x*y", 3, 3]

checkio([[2, 2, [4, 1]], [3, 3, [6, 1]]]) # ["x+y", 5, 5]

What should I return as x and y?

your guess as an expression string

Any syntax requirements?

Are 'x+y', 'x + y', '(x+y)', 'y+x', '(x + x) - (x + y)' same functions?