To compare two arbitrary revisions of the <del>current VC fileset,</del> <ins>whole trees,</ins> call <del>@code{vc-diff}</del> <ins>@code{vc-root-diff}</ins> with a prefix argument: @kbd{C-u C-x v <del>=}.</del> <ins>D}.</ins>  This prompts for two revision IDs (@pxref{VCS Concepts}), and displays a diff between those versions of the <del>fileset.  This will not work reliably for multi-file VC filesets, if the version control system is file-based rather than changeset-based (e.g., CVS), since then revision IDs for different files would</del> <ins>entire version-controlled directory trees (RCS, SCCS, CVS, and SRC do</ins> not <del>be related in any meaningful way.</del> <ins>support this feature).</ins>