Seems obvious in retrospect.
Revisions and Commits
My best idea for the behavior of arc weld is to work like cat, except overlap the characters of the leading and trailing lines of each file randomly (for each <line, character> position, randomly select the character from either the first or second file, creating an alloyed region where both files are equally represented). This makes sense with the weld command thematically, and has the distinct benefit of being utterly useless.
For example, if you have this file:
XXXXX XXXXX XXXXX XXXXX XXXXX
...and this file:
YYYYY YYYYY YYYYY YYYYY YYYYY
...and you ran cat x.txt y.txt, you'd get this output:
XXXXX XXXXX XXXXX XXXXX XXXXX YYYYY YYYYY YYYYY YYYYY YYYYY
But if you ran arc weld x.txt y.txt, imagine getting this output!
XXXXX XXXXX XXXXX XYYXY YXYYX YYYYY YYYYY YYYYY
The files have been welded together! Amazing!