VMS Help PASCAL, Predeclared Routines, Parameter, PRESENT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  The PRESENT function returns  a  Boolean  value  that  indicates
  whether  the  actual  argument  list  of  a  routine contains an
  argument that corresponds to a formal parameter.   (The  PRESENT
  function  is usually used to supply a default value or to take a
  default action when the argument for a parameter is omitted.)

  Syntax:

     PRESENT (parameter-name)

  The 'parameter-name' parameter is the name of a formal parameter
  with  the  TRUNCATE attribute.  The 'parameter-name' must be the
  name of a formal parameter of the function from which PRESENT is
  called,  or  from  a  subroutine of that function.  The function
  result indicates whether the argument  list  of  the  containing
  routine   specifies  an  actual  argument  corresponding  to  an
  optional parameter.

  Parameters that do not have the TRUNCATE attribute and  also  do
  not follow a parameter with the TRUNCATE attribute in the formal
  parameter list, are allowed; in their case, the PRESENT function
  always returns TRUE.

  Default parameters are considered to be present in the  argument
  list, and the PRESENT function returns TRUE when passed the name
  of a parameter with a default value.