code: function(expr, context, subIntegral) {
        if(expr.type === "OperatorNode" && (expr.op === '+' || expr.op === '-')) {
          var childInts = expr.args.map(function(expr) {
            return subIntegral(expr, context, "sum rule");
          });
  
          if(childInts.every(function(n) { return n; })) {
            return new OperatorNode(expr.op, expr.fn, childInts);
          }
        }
      }
docs: integral(+/- f(x), x) = +/- integral(f(x), x) and integral(f(x) +/- g(x), x) = integral(f(x), x) +/- integral(g(x), x)