sebres' test case in 993edf2e640bee9645d42b48866ffdc61dbd40a2 (core-8-6-branch as of now)

% set a [list "#1st" "#2nd"]; set b [list "#3rd" "#4rt"];                   dict set d [concat $a $b] "val-A"
{{#1st} #2nd {#3rd} #4rt} val-A
% set a [list "#1st" "#2nd"]; set b [list "#3rd" "#4rt"]; string length $b; dict set d [concat $a $b] "val-B"
{{#1st} #2nd {#3rd} #4rt} val-B
%