{

#if DEBUG

std::cout << "diff: " << toString(exp) << "\t" << toString(var) << std::endl;

#endif // DEBUG

using namespace matchit;

Id<Sum> iS;

Id<Product> iP;

Id<ExprPtr> iEBase, iEExp, iENat;

Id<ExprPtr> iE;

Id<PieceWise> iPieceWise;

return match(exp)(

pattern | asDouble = expr(0_i),

pattern | var = expr(1_i),

pattern | some(as<Symbol>(_)) = expr(0_i),

pattern | some(as<Sum>(iS)) = [&]

{ return std::accumulate((*iS).begin(), (*iS).end(), 0_i, [&](auto &&sum, auto &&e)

{ return sum + diff(e.second, var); }); },

pattern | some(as<Product>(iP)) = [&]

{ return std::accumulate((*iP).begin(), (*iP).end(), 0_i, [&](auto &&sum, auto &&e)

{ return sum + diff(e.second, var) / e.second * exp; }); },

pattern | some(as<Power>(ds(iEBase, iEExp))) = [&]

{

auto const &n = *iEExp;

auto const &u = *iEBase;

return n * (u ^ (n - 1_i)) * diff(u, var);

},

pattern | some(as<Sin>(ds(iE))) = [&]

{ return sin(pi / 2_i - *iE) * diff(*iE, var); },

pattern | some(as<PieceWise>(iPieceWise)) = [&]

{

PieceWise result;

for (auto const &e : *iPieceWise)

{

result.push_back({diff(e.first, var), e.second});

}

return makeSharedExprPtr(std::move(result));

},

pattern | _ = [&]

{

throw std::runtime_error{"No match in diff!"};

return 0_i;

});