I use the following idiom for string insertion. It works reasonably well when the integer width is small, 1 or 2 bytes.
But first, let's change the problem slightly get rid of the "e", then
insert "www" after the "l" in "alx". This solution is for origin 1.
a←'alx'
b←'www'
(a,b)[⍋(⍳⍴a),(⍴b)⍴2]
⍳⍴a is 1 2 3
2 is the position of the "l". The "www" will be inserted after this position.
(⍴b)⍴2 is 2 2 2
⍋(⍳⍴a),(⍴b)⍴2 is 1 2 4 5 6 3
This technique works very well with multiple insertions.
