public void prependNewSibling() {
Argument current = getSelection().getCurrent();
Selection s = getSelection();
Cursor c = s.getCursor();
Argument replacement = document.parseArgument(getLine());
boolean ok = current.isBranch() || s.variableTokenToBranch(knownArguments);
if (!ok) return;
// TODO: What about current.isRoot()?
if (replacement!=null) {
s.replace(replacement);
}