xmodmap -e "add Control = Control_R" fails (so does "add Mod1 = Meta_L Meta_R")

[x11@…]

macbookpro mid-2010, macosx-10.6.4, xquartz-2.5.3 (with option keys send Alt_L and Alt_R in preferences).

i'm getting errors when trying to set modifier keys using xmodmap.

the following xmodmap commands make:

• the left alt key become Meta_L (instead of Alt_L), and
• the right alt key becomes Control_R (instead of Alt_R).
• then the control modifiers are cleared and set to the left and right control keys, and
• the mod1 modifiers are cleared and set to be the left and right meta keys.

 keycode 66 = Meta_L NoSymbol Meta_L
 keycode 69 = Control_R NoSymbol Control_R
 clear Control
 add Control = Control_L Control_R
 clear Mod1
 add Mod1 = Meta_L Meta_R

i get an error (BadValue (integer parameter out of range for operation)) in the add commands with Control_R and Meta_L and Meta_R. the Control_L on its own is accepted.

it works with the X11.app that comes with macosx-10.6.4 (xquartz-2.3.5).