Make add explicit

src/types/number/serialize.js

@@ -13,7 +13,7 @@ const serializeNodes = function(values, nodes) {
 }
 
 const hasAdd = function({ add }) {
-  return add
+  return add === true
 }
 
 const sum = function(memo, number) {

src/types/octal/parse.js

@@ -42,7 +42,7 @@ const invertAdd = function(node) {
 }
 
 const hasAdd = function({ add }) {
-  return add
+  return add === true
 }
 
 // =octal means that some permissions are +, others -

src/types/octal/serialize.js

@@ -34,7 +34,7 @@ const serializeOperator = function({ nodes }) {
 }
 
 const isRemoved = function({ add }) {
-  return !add
+  return add !== true
 }
 
 // Re-use `number` serialization logic, then stringify to an octal number

src/types/stat/serialize.js

@@ -25,7 +25,7 @@ const getAddedNodes = function({ nodes }) {
 }
 
 const hasAdd = function({ add }) {
-  return add
+  return add === true
 }
 
 // Returns either `-` or the permission character