Setting the __mode metatable field

[David Jones <drj@...>]

t={}
setmetatable(t,{})
getmetatable(t).__mode='v'

Does that count as violating the restriction that "you should not 
change the value of its [the metatable's] field __mode"?  This 
restriction is documented in section 2.9.2, 
http://www.lua.org/manual/5.0/manual.html#2.9.2 .  In other words, if 
the __mode field does not exist, does creating one count as changing 
its value.  Can you change the value of a non-existant field?  Do all 
fields really exist, it's just that most of them are nil?

Also, "setmetatable(x, y)" appears to evaluate to x.  Should this be 
documented?

David Jones