Say that a natural number is alternating-even iff all the digits in odd position are odd, and all those in even position are even (assuming that the position of the rightmost digit is 0).
For instance, 8, 72 and 1234 are alternating-even, while 3, 51 and 8234 are not.
Define a function with type:
alt_even: int -> boolsuch that alt_even x evaluates to true if and only if x is alternating-even.