  consider an HTML lexer and a CSS lexer. Either lexer stands alone for styling
    their respective HTML and CSS files. However, CSS can be embedded inside
    HTML. In this specific case, the CSS lexer is the &ldquo;child&rdquo; lexer with the HTML
    lexer being the &ldquo;parent&rdquo;. Now consider an HTML lexer and a PHP lexer. This
    sounds a lot like the case with CSS, but there is a subtle difference: PHP
    <em>embeds itself into</em> HTML while CSS is <em>embedded in</em> HTML. This fundament