fmul method
Implementation
void fmul( MPData ret, MPData a, MPData b, int prec ){
a = clone( a );
b = clone( b );
mul( ret, a, b );
int p = getPrec( a ) + getPrec( b );
int n = (p - (prec + digit)) ~/ digit;
if( n > 0 ){
p -= n * digit;
_fdiv( ret, n );
}
_setPrec( ret, p );
}