You can judge a chorus by its unison singing, and, by this criterion, the chamber ensemble that performed at the Library of Congress last night was quite expert. The occasion was the opening of this year's Inter-American Music Festival, and the 19-voice chorus conducted by the University of Maryland's Paul Traver was aptly called the "Coro del Festival." This may be nominally an adhoc group, but the blend and clarity they achieved was a dead giveaway. They've sung together a lot and Traver has trained them well.

For a progaram, they picked bits and pieces from the large available repertoire of music of the Americas, and a group of folk-song arrangements by Bartok, an adopted son, perhaps.

There were the requisite delights by Billings and Belcher, Barber's splendid "Reincarnations" highlighted by a gorgeous performance of "Anthony O Daly," a shaky reading of Bernstein's "The Lark" and some pleasant madrigals by Diemer. In a more daring mode, there were two works for chorus and tape, one by Bassett with nice vocal writing but superfluous tape noises, and a piece of garbarge called "In the Beginning of Creation" by Pinkham, who usually knows better.

A beautifully sung "Magnificat" by Franco and music by Villa-Lobos and Sandi gave the concert its international flavor. Barber and Bartok walked off with the honors, however.