Index: src/pkg/os/user/user.go
===================================================================
--- a/src/pkg/os/user/user.go
+++ b/src/pkg/os/user/user.go
@@ -9,15 +9,30 @@
 	"strconv"
 )
 
+
 // User represents a user account.
 type User struct {
-	Uid int // user id
-	Gid int // primary group id
+	// For POSIX systems
+	Uid int // user id
+	Gid int // primary group id 
+
+	// For Windows
+	SID string // user system id
+
+	// Common to all systems
 	Username string
-	Name string
+	Name string // User's name if available
 	HomeDir string
 }
 
+// UnknownUserSIDError is returned by LookupSID when
+// a user cannot be found.
+type UnknownUserSIDError string
+
+func (e UnknownUserSIDError) String() string {
+	return "user: unknown user SID " + string(e)
+}
+
 // UnknownUserIdError is returned by LookupId when
 // a user cannot be found.
 type UnknownUserIdError int
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