Is it acceptable for one header file to #include another?

It's a question of style, and thus receives considerable debate. Many people believe that ``nested #include files'' are to be avoided: the prestigious Indian Hill Style Guide disparages them; they can make it harder to find relevant definitions; they can lead to multiple-definition errors if a file is #included twice; they can lead to increased compilation time; and they make manual Makefile maintenance very difficult. On the other hand, they make it possible to use header files in a modular way (a header file can #include what it needs itself, rather than requiring each #includer to do so); a tool like grep (or a tags file) makes it easy to find definitions no matter where they are; a popular trick along the lines of:
#ifndef HFILENAME_USED
#define HFILENAME_USED
...header file contents...
#endif

(where a different bracketing macro name is used for each header file) makes a header file ``idempotent'' so that it can safely be #included multiple times; a clever compiler can avoid expending any more time on later instances of an already-included header; and automated Makefile maintenance tools (which are a virtual necessity in large projects anyway; andle dependency generation in the face of nested #include files easily.