I need to find all *.txt files in the directory, like
find ./ -iname "*txt"
I wanted an Emacs solution, so I found the multi-command icicle-locate-file-other-window.
Its documentation tells me to type a multi-completion pattern to match
f.* C-M-j some text .* in the file
Nothing happens when I press C-M-j. What command is this bound to?
I also know about find-name-dired, but I want to try this Icicles thingy ;).
icicle-locate-file(-other-window) allows (but doesn't require) multi-completion input matching.
In your example you apparently just want to find all files whose ''names'' match *.txt, so you don't provide a second pattern, to match file content, at all. You just type .*\.txt S-TAB, that is, a regexp matching the file name, followed by S-TAB for apropos (regexp-matching) completion.
But if you ''do'' want to also match file ''content'' then you use C-M-j to tell Icicles that your first, file-name-matching pattern is finished and you next will type a file-content-matching pattern.
(Unless you've set option icicle-show-Completions-initially-flag to non-nil, you need to tell Emacs to show completions, at least initially, using TAB or S-TAB. And if you do want to match more than just the first part then use S-TAB.)
To match multiple parts, you use C-M-j after each input pattern for a part. What that does is insert a string that separates the patterns - ''nothing more''. (C-M-j is bound during minibuffer completion to command icicle-insert-list-join-string.)
By default, the separator string is the two chars ^G (Control G) followed by ^J (Control J, aka ''newline''). You could enter the string chars yourself, if you wanted, instead of using C-M-j -- to insert a Control-G char you need to use C-q C-g. (In Icicles, C-j/newline is self-inserting.)
This odd separator string is the default because (1) the newline char gives you a new minibuffer line for each pattern and (2) it's very unlikely that any of your patterns will contain such a string. (You don't see the ^G character because it's hidden.) Contrast this with completion frameworks that use SPC, which doesn't let you use space chars in patterns. You can customize the separator string, using option icicle-list-join-string. (And some Icicles commands, such as icicle-search, bind it to a simpler string, such as "^I" (just a TAB char) or ": ".
This part of the Nutshell View of Icicles gives a simple overview of using multicompletion. This topic presents multicompletion in detail. This topic tells you about commands, such as icicle-locate-file, that read file names. And this topic tells you about using multicompletion with such commands, to match both file names and file content.