To use K terminology, fuse is a dyad which takes a delimiter as its left argument:
"\n" fuse ("hello","world","etc") "hello\nworld\netc" ":ANYTHING:" fuse ("hello","world","etc") "hello:ANYTHING:world:ANYTHING:etc"
And you can handle even fancier cases with a recursive "format":
(list "ITEM<%s>") format ("hello","world","etc") ("ITEM<hello>","ITEM<world>","ITEM<etc>") ("\n","ITEM<%s>") format ("hello","world","etc") "ITEM<hello>\nITEM<world>\nITEM<etc>"