XLE uses a form of Optimality Theory that allows the grammar writer to indicate that certain constructions are dispreferred. This means that the grammar can parse sentences that are ill-formed in specific ways. For instance, the grammar may be able to parse sentences with subject-verb disagreements (e.g. "They walks"). Because the ill-formed analyses are dispreferred, they are not included in the output if there are well-formed analyses present. In addition, XLE has the capability of producing well-formed fragments if the grammar doesn't cover the entire input. The combination of these two capabilities makes XLE robust in the face of ill-formed inputs and shortfalls in the coverage of the grammar.