  We propose USim, a semantic measure for Grammatical Error Correction (GEC)
that measures the semantic faithfulness of the output to the source, thereby
complementing existing reference-less measures (RLMs) for measuring the
output's grammaticality. USim operates by comparing the semantic symbolic
structure of the source and the correction, without relying on manually-curated
references. Our experiments establish the validity of USim, by showing that (1)
semantic annotation can be consistently applied to ungrammatical text; (2)
valid corrections obtain a high USim similarity score to the source; and (3)
invalid corrections obtain a lower score.
