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CRTL, fdim, Return Values
 *Conan The Librarian 

    n                  Upon success, the positive difference value.
    HUGE_VAL           If x - y is positive and overflows; errno is
                       set to ERANGE.
    0                  If x - y is positive and underflows; errno is
                       set to ERANGE.
    NaN                x or y is NaN; errno is set to EDOM.
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