Modula-3: Predeclared opaque types

## Predeclared opaque types

The language predeclares the two types:

        TEXT  <: REFANY
        MUTEX <: ROOT

which represent text strings and mutual exclusion semaphores, respectively. These are opaque types. Their properties are specified in the required interfaces `Text` and `Thread`.

    Last modified on Wed Apr  3 09:39:11 PST 1996 by heydon
         modified on Mon Apr 18 14:01:18 PDT 1994 by kalsow