[NTG-context] lbrack?

This program: \setupbodyfont[lmodern] \starttext \type{$\lbrace$} <$\lbrace$>\crlf \type{$\lbrack$} <$\lbrack$>\crlf \stoptext This does typeset a left brace but chokes on \lbrack. I took my copy of the TeX Book and find \brack as one of the defined delimiters in chapter 17. How come? ConTeXt ver: 2010.03.20 22:59 MKIV fmt: 2010.3.21 int: english/ english fonts : preloading latin modern fonts bodyfont : 12pt rm is loaded language : language en is active systems : begin file test.tex at line 2 ! Undefined control sequence. <recently read> \lbrack l.4 \type{$\lbrack$} <$\lbrack $>\crlf ? Hans van der Meer