The #include directive instructs the preprocessor to insert the
  contents of the specified file or module into the program.  An
  #include directive can have one of three forms:

       #include "filespec"
       #include <filespec>
       #include module-name

  The first two forms are ANSI-compliant methods of file inclusion
  and are therefore more portable.  In these forms, .h is the default
  file type, unless the compiler is instructed to supply no default
  type (that is, a type of just ".") by the
  /ASSUME=NOHEADER_TYPE_DEFAULT qualifier.

  The third form is specific to OpenVMS systems for specifying the
  inclusion of a module from a text library, and is not generally
  needed or recommended because the ANSI forms also cause the text
  libraries to be searched.

  For the order of search, see /INCLUDE_DIRECTORY.

  There is no defined limit to the nesting level of #include files
  and modules.