A meaningless line is removed.

Another possible fix is to insert following line after the deleted line:

    vim.command(vc)

The deleted line has no effect with out this line. As far as I checked
however, s:swank_action is always used in combination with
swank_response method and slimv.vim side always clear the variable
before calling the method. Therefore, I suppose that the better fix is
delete the line.
Echomode is added to control where the output should be printed.

For now, all the behavior is hard coded and not extensible at all. The
keymap <Leader>f behaves identical to <Leader>d except the output
buffer. The result is printed to the flipflop buffer.
[Tiny] The result of describe-symbol should be printed to REPL buffer.

I remember that SLIMV used to behave in this manner and I prefer this
style.