Y_LAUNCH the directory containing the Yorick executable
Y_SITE Yorick's "site directory"
Y_LAUNCH is set by compiled code when Yorick starts and should never
Y_SITE is set to a default built-in value by compiled code at startup,
but may be modified in the file Y_LAUNCH/paths.i to allow for
ports of Yorick binary executables.
builtin function, documented at i0/paths.i line 82