Tests the value of an expression and, depending on the syntax
    specified, executes the following:

    o  One command following the THEN keyword if the expression is
       true

    o  Multiple commands following the $THEN command if the
       expression is true

    o  One or more commands following the $ELSE command if the
       expression is false

    Format

      $ IF  expression THEN [$] command

      or

      $ IF  expression

      $ THEN  [command]

      command

         .
         .
         .

      $ [ELSE]  [command]

      command

         .
         .
         .

      $ ENDIF

                                   NOTE

       VSI advises against assigning a symbolic name that is already
       a DCL command name. VSI especially discourages the assignment
       of symbols such as IF, THEN, ELSE, and GOTO, which can
       affect the interpretation of command procedures.