docs: public String getPrefix(INode prefixNode) {
  		if (prefixNode instanceof ILeafNode) {
  			if (((ILeafNode) prefixNode).isHidden() && prefixNode.getGrammarElement() != null)
  				return "";
  			return getNodeTextUpToCompletionOffset(prefixNode);
  		}
  		StringBuilder result = new StringBuilder(prefixNode.getTotalLength());
  		doComputePrefix((ICompositeNode) prefixNode, result);
  		return result.toString();
  	}