(defun tex-view ()
"Preview the last `.dvi' file made by running TeX under Emacs.
This means, made using \\[tex-region], \\[tex-buffer] or \\[tex-file].
-The variable `tex-dvi-view-command' specifies the shell command for preview."
+The variable `tex-dvi-view-command' specifies the shell command for preview.
+You must set that variable yourself before using this command,
+because there is no standard value that would generally work."
(interactive)
+ (or tex-dvi-view-command
+ (error "You must set `tex-dvi-view-command'"))
(let ((tex-dvi-print-command tex-dvi-view-command))
(tex-print)))