expression

   One of the attributes of an occurrence. The expression attribute
   tells you whether the occurrence is one that you the user placed
   in the source code, or is one which the compiler created for
   the user. An occurrence can be either explicit or implicit.
   Explicit occurrences are those placed by the user in the source
   code; implicit occurrences are those created by the compiler on
   behalf of the user. For example, in the following FORTRAN program,
   there are three references to i which are explicit. There is one
   implicit declaration of i which is created by the FORTRAN compiler
   when it sees the first reference to i.

     program test
     i = 0
     i = i + 1
     end