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README file: <IDL_Directory>
               external
                 call_external
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The call_external directory contains examples of calling code
written in compiled languages like C or Fortran via the IDL
CALL_EXTERNAL function. It contains the following subdirectories:

  - C 
    The C examples for CALL_EXTERNAL. 

  - Fortran
    The Fortran examples for CALL_EXTERNAL.