src/Pure/PIDE/command_span.scala

     def name: String =
       kind match { case Command_Span(name, _) => name case _ => "" }
 
     def position: Position.T =
       kind match { case Command_Span(_, pos) => pos case _ => Position.none }
+
+    def keyword_pos(start: Token.Pos): Token.Pos =
+      kind match {
+        case _: Command_Span =>
+          (start /: content.iterator.takeWhile(tok => !tok.is_command))(_.advance(_))
+        case _ => start
+      }
 
     def is_begin: Boolean = content.exists(_.is_begin)
     def is_end: Boolean = content.exists(_.is_end)
 
     def length: Int = (0 /: content)(_ + _.source.length)