So one of the books I checked out from Hillman this last time in my usual grab-whatever-has-a-sexy-sounding-title habit was Kayne's "The Asymmetry of Syntax". It's crazy stuff: he is claiming that every language is always head-initial and all "head-final" aspects are completely illusory and explainable by other stuff. Trying to machete my way through the dense foliage of definitons, I googled for "c-command syntax x bar" and got this paper, which at least has the definition of "c-commands".