Wait for a Key Press in a Terminal
Description
Wait for a single key press at the 'R' prompt.
This works in terminals, but does not currently work
in the 'Windows' 'GUI', the 'OS X' 'GUI' ('R.app'),
in 'Emacs' 'ESS', in an 'Emacs' shell buffer or in
'R Studio'. In these cases 'keypress' stops with an
error message.