I just finished up Problem 10; given my earlier work on Problem 7, it was trivial to adapt it and arrive at the following program.

```
"""Solves Problem 10 for Project Euler."""
import math
def is_prime(candidate, known_primes):
"""Determines whether candidate is prime by trial division using \
known_primes.
For this function to work, known_primes *must* be accurate.
"""
last_possible = math.sqrt(candidate)
for current_prime in known_primes:
if current_prime > last_possible:
break
if not candidate % current_prime:
return False
return True
def primes_generator(upper_bound):
"""A generator for all the primes < upper_bound."""
candidates = xrange(2, upper_bound)
primes = []
for n in candidates:
if is_prime(n, primes):
primes.append(n)
yield n
def problem_10():
"""Sum the primes less than 2 million."""
return sum(primes_generator(2000000))
if __name__ == '__main__':
print problem_10()
```

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