code: function(expr, context, subIntegral) {
        if(expr.type === 'FunctionNode' && expr.args[0].equals(context.variable)) {
          switch(expr.name) {
            case "sin":
              return new OperatorNode('-', 'unaryMinus', [
                new FunctionNode("cos", [context.variable])
              ]);
            case "cos":
              return new FunctionNode("sin", [context.variable]);
            case "tan":
              //log(abs(sec(x)))
              return new FunctionNode('log', [
                new FunctionNode('abs', [
                  new FunctionNode('sec', [context.variable])
                ])
              ]);
            case "sec":
              //log(abs(sec(x) + tan(x)))
              return new FunctionNode('log', [
                new FunctionNode('abs', [
                  new OperatorNode('+', 'add', [
                    new FunctionNode('sec', [context.variable]),
                    new FunctionNode('tan', [context.variable])
                  ])
                ])
              ]);
            case "csc":
              //log(abs(sec(x) + tan(x)))
              return new FunctionNode('log', [
                new FunctionNode('abs', [
                  new OperatorNode('-', 'subtract', [
                    new FunctionNode('csc', [context.variable]),
                    new FunctionNode('cot', [context.variable])
                  ])
                ])
              ]);
            case "cot":
              //log(abs(sec(x) + tan(x)))
              return new FunctionNode('log', [
                new FunctionNode('abs', [
                  new FunctionNode('sin', [context.variable])
                ])
              ]);
            default:
              return null;
          }
        }
      }
docs: integral(sin(x), x) = -cos(x) integral(cos(x), x) = sin(x) integral(tan(x), x) = log(abs(sec(x))) integral(sec(x), x) = log(abs(sec(x) + tan(x))) integral(csc(x), x) = log(abs(csc(x) - cot(x))) integral(cot(x), x) = log(abs(sin(x)))