+If @code{comint-use-prompt-regexp} is @code{nil} (the default), copy
+the old input command that you click on, inserting the copy at the end
+of the buffer (@code{comint-insert-input}). If
+@code{comint-use-prompt-regexp} is non-@code{nil}, or if the click is
+not over old input, just yank as usual.