[NTG-context] \coloneqq

I'd like to use a colon-equals sign symbol to indicate a definition. According to The Comprehensive LaTeX Symbol List, this symbol is available in the txfonts (LaTeX) package. However, it seems that the txfonts support in ConTeXt doesn't contain that symbol. And, in any case, a search for \coloneqq on the wiki gave no results. Perhaps the symbol I want isn't called coloneqq in ConTeXt, or perhaps the symbol is currently missing. How can I get this symbol in my ConTeXt document? Thanks for your help, Jesse -- Jesse Alama (alama@stanford.edu)