VMS Help FORTRAN, /CHECK, ASSERTIONS  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Enables or disables assertion checking.  Assertions give the
  compiler information that can resolve ambiguous data dependences.

  If /CHECK=ASSERTIONS is specified, the compiler will generate the
  appropriate run-time code to evaluate the asserted expression.  If
  the assertion tests false at run time, the program is stopped and
  an error message is issued.

  If /CHECK=NOASSERTIONS is specified, the compiler is free to use
  the information in the assertions in its optimizations and
  transformations, but no code is generated to test the validity of
  the assertions at run time.  Assertions can be coded as statements
  or general compiler directives:

     CDEC$ ASSERT (e)
           ASSERT (e)

     Where e is a logical expression.