add SOURCE-ID

README.md

@@ -15,7 +15,7 @@ Alee Forth uses the [Forth 2012 test suite](https://github.com/gerryjackson/fort
 **Missing** core extension words:
 
 ```
-PARSE-NAME REFILL RESTORE-INPUT S\" SAVE-INPUT SOURCE-ID
+PARSE-NAME REFILL RESTORE-INPUT S\" SAVE-INPUT
 ```
 
 ## Building

forth/core-ext.fth

@@ -74,3 +74,4 @@
             rot dup >r c! r> char+ swap repeat
             2drop r> tuck - ;
 
+: source-id _source @ _begin < if 0 else -1 then ;