This model demonstrates the use of function closures inside a CAL actor.
The PrimeSieve actor uses nested function closures to realize the Sieve of Eratosthenes,
a method for finding prime numbers. Its state variable, "filter," contains the current filter
function. If it is "false" a new prime number has been found, and a new filter function
will be generated.
The PrimeSieve actor expects an ascending sequence of natural numbers, starting from 2,
as input.