# Compose an array, then 'tap' it to begin iterating over its values;
# our bounds are 2 .. X-2, where X is last digit we want to test.
# The last operation performed, the select, accepts only non-nil
# values.
Array.new( (ARGV[0] || 1000).to_i - 2 ){|i| i + 2 }.tap {|it|
    it.each_with_index {|v, i|
        next unless v

        # We actually only test slices of it, since we already know that
        # any given number is divisible by itself.
        it[ i + 1 .. it.length - 1 ].each {|n|
            next unless n

            # If it's a factor, remove it!
            it.delete( n ) if n % v == 0
        }
    }
}.select {|r| !!r }