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`log', `logf'--natural logarithms

     #include <math.h>
     double log(double X);
     float logf(float X);
Return the natural logarithm of X, that is, its logarithm base e (where
e is the base of the natural system of logarithms, 2.71828...).  `log'
and `logf' are identical save for the return and argument types.
   You can use the (non-ANSI) function `matherr' to specify error
handling for these functions.
Normally, returns the calculated value.  When X is zero, the returned
value is `-HUGE_VAL' and `errno' is set to `ERANGE'.  When X is
negative, the returned value is `-HUGE_VAL' and `errno' is set to
`EDOM'.  You can control the error behavior via `matherr'.
`log' is ANSI, `logf' is an extension.