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How do I emulate a keypress inside a Vim function?
function BigScrollUp()
  let count = 20
  while count > 0
    "Press" CTRL-Y <-- how do I emulate this?
    sleep 5m
    count -= 1
  endwhile
endfunction

" Press CTRL-Y:
normal <Ctrl+v><Ctrl+y>

feedkeys({string} [, {mode}])               *feedkeys()*
        Characters in {string} are queued for processing as if they
        come from a mapping or were typed by the user.  They are added
        to the end of the typeahead buffer, thus if a mapping is still
        being executed these characters come after them.
        The function does not wait for processing of keys contained in
        {string}.
        To include special keys into {string}, use double-quotes
        and "..." notation |expr-quote|. For example,
        feedkeys("<CR>") simulates pressing of the <Enter> key. But
        feedkeys('<CR>') pushes 5 characters.
        If {mode} is absent, keys are remapped.
        {mode} is a String, which can contain these character flags:
        'm' Remap keys. This is default.
        'n' Do not remap keys.
        't' Handle keys as if typed; otherwise they are handled as
            if coming from a mapping.  This matters for undo,
            opening folds, etc.
        Return value is always 0.

call feedkeys("<C-Y>")