Assertion: white pebble | /r/HasProperty | placed at any vertex
Facets:
Source sentences:
text: It also adds white pebbles, which may be placed at any vertex at will, but can only be removed if all the vertex's immediate descendant vertices are also pebbled.
text: It also adds white pebbles, which may be placed at any vertex at will, but can only be removed if all the vertex's immediate descendant vertices are also pebbled.