* profiles from the project's POM and all its parent POMs as well as from external sources like the
     * {@code settings.xml}. The profile identifiers are grouped by the identifier of their source, e.g.
     * {@code <groupId>:<artifactId>:<version>} for a POM profile or {@code external} for profiles from the
     * {@code settings.xml}.
     *

     * We track the "meta iterators," the iterators-of-iterators, below.  Usually, topMetaIterator
     * is the only one in use, but if we encounter nested concatenations, we start a deque of
     * meta-iterators rather than letting the nesting get arbitrarily deep.  This keeps each
     * operation O(1).
     */

    @CheckForNull private Iterator<? extends Iterator<? extends T>> topMetaIterator;