Method _walk

chapter17/src/main/java/com/seaofnodes/simple/node/Node.java:630–638

( Function<Node,E> pred )

    }

    private <E> E _walk( Function<Node,E> pred ) {
        if( WVISIT.get(_nid) ) return null; // Been there, done that
        WVISIT.set(_nid);
        E x = pred.apply(this);
        if( x != null ) return x;
        for( Node def : _inputs  )  if( def != null && (x = def._walk(pred)) != null ) return x;
        for( Node use : _outputs )  if( use != null && (x = use._walk(pred)) != null ) return x;
        return null;
    }

    /**
     * Debugging utility to find a Node by index