Checking a probability table

You will turn survey counts into a probability table, test whether a table is a real distribution, and add up the probabilities of "at least $k$". The checker uses the made-up survey of brothers and sisters.

The idea. A table is a dictionary that maps each value to its probability. The probability of a value is its count divided by the total. A real distribution has no negative probability, and its probabilities add up to exactly $1$.

Your tasks.

1. Finish table_from_counts(counts). Divide each count by the total, with Fraction so the answer is exact.
2. Finish is_distribution(pmf). Return True only if every probability is at least $0$ and the total is $1$.
3. Finish at_least(pmf, k). Add the probabilities of every value that is $k$ or more.