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`scalbn', `scalbnf'--scale by integer

     #include <math.h>
     double scalbn(double X, int Y);
     float scalbnf(float X, int Y);
`scalbn' and `scalbnf' scale X by N, returning X times 2 to the power
N.  The result is computed by manipulating the exponent, rather than by
actually performing an exponentiation or multiplication.
X times 2 to the power N.
Neither `scalbn' nor `scalbnf' is required by ANSI C or by the System V
Interface Definition (Issue 2).