c-x c-v doesn't hack major mode from extension

[zvona@MIT-OZ]

In zwei in Release 4.3, LMFS 30.42, Experimental TimeKit 7.0,
Experimental Music 2.0, Experimental Pitch 4.0,
Experimental Enharm-Pitch 2.0, Experimental Harmony 2.0,
Experimental Staffs 2.0, Experimental Chroma 2.0, Experimental Play 26.0,
Experimental Compose 13.0, site configuration 33, Music 2.0, on Lisp Machine Four:

do c-x b gruddle c-cr c-x x-v probbix.lisp.  The buffer stays in 
fundamental mode, rather than going into lisp mode.