integral(+/- f(x), x) = +/- integral(f(x), x) and integral(f(x) +/- g(x), x) = integral(f(x), x) +/- integral(g(x), x)
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);
        }
      }
    }