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  The Title VII antiretaliation provision has two clauses, making it 'an unlawful employment practice for an employer to discriminate against any of his employees . . . [1] because he has opposed any practice made an unlawful employment practice by this subchapter, or [2] because he has made a charge, testified, assisted, or participated in any manner in an investigation, proceeding, or hearing under this subchapter.' The first is known as the 'opposition clause,' the other as the 'participation clause.' The opposition clause makes it 'unlawful . . . for an employer to discriminate against any . . . employee . . ...