[NTG-context] metapost question

I cannot find the answer to the following question in Knuth's Metafont book. The following definition with a trailing text argument: def mydef (expr a, b, c) text modifier = if : else: fi enddef; How can I do the switch on an empty or nonempty modifier argument? It is not a string because calling its length fails with an error. I must do the switch, because "fill modifier" with an empty "modifier" gives horrible results. Usage would be calls like: mydef(1,2,3) withcolor green; % with modifier text mydef(1,2,3); % without modifier text Thanks in advance. Hans van der Meer