The Free On-line Dictionary of Computing (30 December 2018):

CLU

    (CLUster) An object-oriented programming language
   developed at MIT by Liskov et al in 1974-1975.

   CLU is an object-oriented language of the Pascal family
   designed to support data abstraction, similar to Alphard.
   It introduced the iterator: a coroutine yielding the
   elements of a data object, to be used as the sequence of
   values in a for loop.

   A CLU program consists of separately compilable procedures,
   clusters and iterators, no nesting.  A cluster is a module
   naming an abstract type and its operations, its internal
   representation and implementation.  Clusters and iterators may
   be generic.  Supplying actual constant values for the
   parameters instantiates the module.

   There are no implicit type conversions.  In a cluster, the
   explicit type conversions 'up' and 'down' change between the
   abstract type and the representation.  There is a universal
   type 'any', and a procedure force[] to check that an object is
   a certain type.  Objects may be mutable or immutable.

   Exceptions are raised using 'signal' and handled with
   'except'.  Assignment is by sharing, similar to the sharing
   of data objects in Lisp.  Arguments are passed by
   call-by-sharing, similar to call-by-value, except that the
   arguments are objects and can be changed only if they are
   mutable.  CLU has own variables and multiple assignment.

   CLU was one of Kamin's interpreters.  clu2c compiled CLU to
   C.  Concurrent CLU was an extension designed to support
   parallel proceses.

   ["CLU Reference Manual", Barbara Liskov et al, LNCS 114,
   Springer 1981].

   E-mail: Paul R. Johnson .

   Versions for Sun and VAX/VMS
   (ftp://pion.lcs.mit.edu/pub/clu/).  Portable version
   (ftp://mintaka.lcs.mit.edu/pub/dcurtis/).

   (1994-12-16)