[NTG-context] Checking existence of a macro (control sequence) by Lua

[context at vivaldi.net]

Hello,

one more question - why macros \Undefined and \DoNothing show "defined" 
although I (tried to) undefine them?

----
\starttext
   \def\MyMacro{Ahoj}
   \def\MyMac#1#2{Something}

   \def\Undefined{}
   \let\Undefined\undefined

   \def\DoNothing{}
   \let\DoNothing\donothing

   \startluacode
     local str = { [true] = "defined", [false] = "undefined" }

     local function whatever(s)
       context.type("\\" .. s)
       context(" is " .. (tokens.defined(s) and "defined" or 
"undefined")) --% Hans' way
       context(" and is " .. str[token.is_defined(s)] .. ".") --% Henri's 
way
       context.par()
     end

     whatever("MyMacro")
     whatever("MyMacroD")
     whatever(" ")
     whatever("-")
     whatever("Undefined")
     whatever("DoNothing")
   \stopluacode

   %\Undefined % Causes "! Undefined control sequence"
\stoptext
----

Gives:

"
\MyMacro is defined and is defined.
\MyMacroD is undefined and is undefined.
\ is defined and is defined.
\- is defined and is defined.
\Undefined is defined and is defined.
\DoNothing is defined and is defined.
"

Lukas