Index: layer_grp.h
===================================================================
--- layer_grp.h	(revision 29702)
+++ layer_grp.h	(revision 29703)
@@ -138,7 +138,8 @@
 
 /* Move lid 1 step towards the front (delta=-1) or end (delta=+1) of the
    layer list of the group. Return 0 on success (even when already reached
-   the end of the list) or -1 on error */
+   the end of the list) or -1 on error.
+   This operation is always undoable. */
 int pcb_layergrp_step_layer(pcb_board_t *pcb, pcb_layergrp_t *grp, pcb_layer_id_t lid, int delta);
 
 /* Return the array index of lid within the grp's lid list or -1 if not on the list */