User blog comment:Josewong/Extensible-E Extension Challenge/@comment-38877333-20191215001232

​​​​#{#+1}# = #{#}#{#}...{#}#{#}# . . . ..
 * 1) {#+2}# = #{#+1}#{#+1}...{#+1}#{#+1}#
 * 1) {#+#}#b = #{#+b}#

So... E10{#+#}10 = E10{#+10}10.