VMS Help PASCAL, Lexical Elements, Identifiers, Predeclared Identifiers  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Predeclared identifiers name data types, symbolic constants  and
  file  variables,  procedures, and functions.  You can redefine a
  predeclared identifier, but, if you do, the original declaration
  becomes unavailable within the block in which you redeclared the
  word.  Examples of predeclared identifiers include ADDRESS, COS,
  ERR, INTEGER, SQR and TRUE.