nl_tokens: Find the common parent of a and b
pl_tokens: function commonParent ( a , b ) { if ( a === b ) { return a ; } else if ( containerOf ( a , b ) ) { return a ; } else if ( containerOf ( b , a ) ) { return b ; } else { // This will happen at some point, since the root is a container of // everything while ( b = b . <mask> ) { if ( containerOf ( b , a ) ) return b ; } } }