that spanned constituent boundaries (as long as the phrase frequency was high).
for each non-terminal node spanning f with its left child spanning f 1 and its right child spanning f 2 , if the non-terminal symbol is str, the reordered strings will be concatenated in order as f = f 1 f 2 , and if the non-terminal symbol is inv, the reordered strings will be concatenated in inverted order as f = f 2 f 1 .