df_scalar

auto pow2( double x ){ return x^^2; }
auto res1 = df_scalar( &pow2, 1 );
auto res2 = df_scalar( &pow2, 3 );
auto res3 = df_scalar( &pow2, -2 );
assert( abs(res1 - 2.0) < 2e-6 );
assert( abs(res2 - 6.0) < 2e-6 );
assert( abs(res3 + 4.0) < 2e-6 );