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 * SimpleShell compiles to POSIX sh, where the output is supposed to be
   human-readable.
 * The syntax is de facto a mixture of sh, C and Haskell.
+
+
+## Variables
+
+* Variables are local to their function or procedure; there are no (nested)
+  variable scopes.
+* All variables have static types.  Same for function and procedure
+  signatures.
+
+
+## Functions
+
+* Functions are pure; they compute a single value from a list of arguments.
+* Compare pure Haskell functions.
+
+
+## Procedures
+
+* Procedures are similar to POSIX sh functions.
+* Procedures take a list of arguments, and return a pair of
+  exit code and return value.
+* Procedures may receive data from stdin and write data to stdout and stderr.