Mark, Thanks for your reply.
You suggest to the port author to have emacs be the default and supply a variant to override the default, say xemacs. But if there is a common library or file that indicates the presence of either, you could use the "lib:" form of dependent specification that would accept either version of emacs. It doesn't check for a port but a library of file. The ports depending on X11 do this so you can either use Apple's X11 or the one from MacPorts. Like this:
lib:libX11.6:XFree86
Am I naive to propose that, in the case of something system-supplied like Emacs, the dependency be omitted? For Mozart it turns out I can override the default (emacs) by setting an environment variable (OZEMACS=xemacs). Everyone can do this, so IMHO the dependency on MacPorts emacs should be dropped. cheers peter