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Can include files be nested?

Yes. Include files can be nested any number of times. As long as you
 use precautionary measures , you can avoid including the same file
 twice. In the past, nesting header files was seen as bad programming
 practice, because it complicates the dependency tracking function of
 the MAKE program and thus slows down compilation. Many of today’s
 popular compilers make up for this difficulty by implementing a
 concept called precompiled headers, in which all headers and
 associated dependencies are stored in a precompiled state.
 Many programmers like to create a custom header file that has
 #include statements for every header needed for each module. This is
 perfectly acceptable and can help avoid potential problems relating to
 #include files, such as accidentally omitting an #include file in a
 module.