code: function createBlockLookup(tree) {
  	const lookup = new Map();
  
  	tree.walk(node => {
  		const classNames = node.classList;
  		if (classNames.length) {
  			// guess best block name from class or use parent’s block name
  			lookup.set(node,
  				find(classNames, blockCandidates1)
  				|| find(classNames, blockCandidates2)
  				|| lookup.get(node.parent)
  			);
  		}
  	});
  
  	return lookup;
  }
docs: Creates block name lookup for each node in given tree, e.g. finds block
  name explicitly for each node
  @param  {Node} tree
  @return {Map}