+<% | OS.SysErr (name, NONE) => %>
+<b>System error</b>: <% Web.html name %>
+<% | OS.SysErr (name, SOME syserr) => %>
+<b>System error</b>: <% Web.html name %>: <% Web.html (OS.errorName syserr) %>: <% Web.htmlNl (OS.errorMsg syserr) %>
+<% | IO.Io {name, function, cause, ...} => %>
+<b>IO error</b>: <% Web.html name %> for <% Web.html function %><br>
+<b>Cause</b>: <% Web.html (Init.explain cause) %><br>
+<b>Tokens</b>: <p><% Web.htmlNl (Init.tokens ()) %></p>