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+# Primes utility
+
+This program can generate primes, check numbers for prime-ness, or filter a list
+of numbers, passing only primes through. All input is on `stdin`.
+
+## Try it
+
+```{sh}
+$ ghc -O2 -dynamic primes
+$ ./primes 20
+```
+
+## Usage
+
+```
+Usage: primes [--check | [--stdin] [NUM_PRIMES]]
+
+  -h, --help  print this message and exit
+  --check     check all numbers given on stdin for primeness
+  --stdin     reads numbers from stdin; copies primes to stdout
+  NUM_PRIMES  only print the first NUM_PRIMES primes;
+              if not given, output is potentially unbounded
+
+If --check is given, the exit code 0 indicates all numbers were
+prime; 1 indicates that at least one non-prime was passed in.
+```