key binding (alt-5 does not work)

[Emanuel Haupt]

I tried to assign the following bindings for switching the workspaces:

--- ~/.themes/ehaupt/xfwm4/keythemerc begins here ---
[...default until here]
workspace_10_key=Alt+0
workspace_11_key=Control+F11
workspace_12_key=Control+F12
workspace_1_key=Alt+1
workspace_2_key=Alt+2
workspace_3_key=Alt+3
workspace_4_key=Alt+4
workspace_5_key=Alt+5
workspace_6_key=Alt+6
workspace_7_key=Alt+7
workspace_8_key=Alt+8
workspace_9_key=Alt+9
--- ~/.themes/ehaupt/xfwm4/keythemerc ends here ---

Since I've got only 10 workspaces I leave workspace_11_key and
workspace_12_key unchanged.

Everything works fine except for 'Alt-5'. If Alt-5 is pressed, nothing
happens. If I assing another binding it will switch as to workspace 5 as
expected. My assumption is that Alt-5 is already used for something I
couldn't figure out. However 'Alt-5' is not defined in
~/.themes/ehaupt/xfwm4/keythemerc for anything else.

What is 'Alt-5' used/reserved for?

Emanuel

Setup:
xfce 4.4.1
FreeBSD 6.2-RELEASE-p5

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