All libraries (header files .h) that are created for c or c++ programs must have code in them to make sure they are not compiled more than once. The use of #ifndef #define and #endif is used for this. The #ifndef means if not defined. If a define is in the top portion of the library file to demarkate the file by a special name, then an ifndef can be used to make sure the code in that library is omitted if the define was alreay seen by the compiler.
Usage:
#ifndef name
#define name
library code
...
#endif
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