[NTG-context] math: using bold charaters for number sets

Dear ConTeXt folks, according to [1] the number sets were originally set using bold characters and only on the blackboard the double strokes were used. ConTeXt defines for example `\naturalnumbers` or `\reals` [2] which are set using double strokes. How can I redefine those to be just bold? Thanks, Paul PS: Does somebody actually use these commands (`\naturalnumbers`, …`) or do you define for example `\N`? [1] http://projekte.dante.de/DanteFAQ/MathematischerFormelsatz#22 [2] http://wiki.contextgarden.net/Blackboard_bold