[fpc-pascal] RemoveDir: Error: Identifier not found

[Jürgen Hestermann]

If I  hit Alt+Up on "RemoveDir" within my program I am moved to
D:\Programme\lazarus\fpc\2.6.1\source\rtl\objpas\sysutils\diskh.inc(21,10)
with the function header declaration but when doing it again
on "RemoveDir" within this file I get:

Error: identifier not found: RemoveDir

so I can't see the implementation of this function.
Is this a bug or some kind of limitation?