sentence1: this approach was pioneered by galley et al with numerous variants in subsequent research , usually referred to as tree-to-tree , tree-to-string and string-to-tree , depending on where the analyses are found in the training data .
sentence2: this approach was pioneered by galley et al , and there has been a lot of research since , usually referred to as tree-to-tree , treeto-string and string-to-tree , depending on where the analyses are found in the training data .