oh wait, actually…

• besides the previously-set $EDITOR take [jupp jstar mcedit ed vi]
  as sorted list, first one found wins
• set -eu safe, and in the lksh part

dot.mkshrc

@@ -1,5 +1,5 @@
 # $Id$
-# $MirOS: src/bin/mksh/dot.mkshrc,v 1.120 2017/08/08 20:52:11 tg Exp $
+# $MirOS: src/bin/mksh/dot.mkshrc,v 1.121 2017/08/08 21:10:21 tg Exp $
 #-
 # Copyright (c) 2002, 2003, 2004, 2006, 2007, 2008, 2009, 2010,
 #		2011, 2012, 2013, 2014, 2015, 2016, 2017
@@ -56,6 +56,13 @@ smores() (
 	done
 )
 
+# customise your favourite editor here; the first one found is used
+for EDITOR in "${EDITOR:-}" jupp jstar mcedit ed vi; do
+	EDITOR=$(\\builtin whence -p "$EDITOR") || EDITOR=
+	[[ -n $EDITOR && -x $EDITOR ]] && break
+	EDITOR=
+done
+
 \\builtin alias ls=ls l='ls -F' la='l -a' ll='l -l' lo='l -alo'
 \: "${HOSTNAME:=$(\\builtin ulimit -c 0; \\builtin print -r -- $(hostname \
     2>/dev/null))}${EDITOR:=/bin/ed}${TERM:=vt100}${USER:=$(\\builtin ulimit \
@@ -617,7 +624,6 @@ done
 #\\builtin export LANG=C LC_CTYPE=$p LC_MEASUREMENT=$p LC_MESSAGES=$p LC_PAPER=$p
 #\\builtin set -U
 
-p=$(\\builtin whence -p jupp) && EDITOR=$p
 \\builtin unset p
 
 \: place customisations above this line