private void unread(char c) throws SAXException {
        rollbackLocation();
        if (readBufferPos > 0) {
            readBuffer[--readBufferPos] = c;
        } else {
            pushString(null, Character.valueOf(c).toString());
        }
    }
Push a single character back onto the current input stream.
<p>
This method usually pushes the character back onto the readBuffer.
<p>
I don't think that this would ever be called with readBufferPos = 0,
because the methods always reads a character before unreading it, but
just in case, I've added a boundary condition.

@param c
           The character to push back.
@see #readCh
@see #unread (char[])
@see #readBuffer
src/nu/validator/gnu/xml/aelfred2/XmlParser.java