concurrency.md

@@ -26,14 +26,15 @@ brief rationale.
   - Use `MVar`s.  A non-concurrent implementation might implement them in terms of `IORef`, for example. 
   - Use STM.  Easier to use, but not entirely trivial to implement, even in a single-threaded implementation, because exceptions have to abort a transaction ([ sample implementation](http://www.haskell.org//pipermail/haskell-prime/2006-March/001108.html)).
 
-- Concurrent foreign calls are required.
+- Concurrent foreign calls are required.  A **concurrent foreign call** allows other Haskell threads to make progress before the foreign call returns.
   **Rationale**: 
 
   - concurrent foreign calls are required to guarantee progress of other Haskell threads when one thread makes a blocking call.
-  - concurrent foreign calls are required for implementing I/O multiplexing, a principle use of concurrency.
+  - concurrent foreign calls are required for implementing I/O multiplexing, a principal use of concurrency.
   - concurrent foreign calls are required to guarantee timely  responsiveness of an interactive application in the presence of long-running foreign calls.
 
-- Concurrent/reentrant foreign calls are required.  Hence,  the
+- Concurrent/reentrant foreign calls are required.  A **reentrant foreign call** is a foreign call that calls a foreign-exported Haskell function. A concurent/reentrant foreign call is both concurrent and reentrant.
+  Hence,  the
   Haskell system must be able to process call-ins from arbitrary
   external OS threads.
   **Rationale**: