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