integral(x, x) = 1/2*x^2
function(expr, context) {
      if(expr.type === "SymbolNode" && expr.name === context.variable.name) {
        return new OperatorNode('*', 'multiply', [
          new OperatorNode('/', 'divide', [
            new ConstantNode(1),
            new ConstantNode(2)
          ]),
          new OperatorNode('^', 'pow', [
            expr.clone(),
            new ConstantNode(2)
          ])
        ]);
      }
    }