let amc12a-2002-p21 = `!u:num->num.
    (u 0 = 4) /\
    (u 1 = 7) /\
    (!n. n >= 2 ==> u (n + 2) = (u n + u (n + 1)) MOD 10)
==>
    (!n. nsum (0..(n-1)) u > 10000 ==> 1999 <= n)
`;;