function _movePrevToken(ctx) {
        if (ctx.pos.ch <= 0 || ctx.token.start <= 0) {
            //move up a line
            if (ctx.pos.line <= 0) {
                return false; //at the top already
            }
            ctx.pos.line--;
            ctx.pos.ch = ctx.editor.getLine(ctx.pos.line).length;
        } else {
            ctx.pos.ch = ctx.token.start;
        }
        ctx.token = ctx.editor.getTokenAt(ctx.pos);
        return true;
    }
@private
moves the current context backwards by one token
@param {editor:{CodeMirror}, pos:{ch:{string}, line:{number}}, token:{object}} ctx
@return {boolean} whether the context changed