Though there was nothing in the least dry about it, last night's program by the Pennsylvania Ballet at the Kennedy Center Opera House was a double-barreled lesson--at once an illustration, in its three ballets, of the major branches of the company's repertory, and a demonstration, in its sharp, stylish and vivacious performances, of the troupe's remarkable strength in each of these tributaries.

George Balanchine's "Square Dance" was a reminder of the company's roots, under its founder and former director Barbara Weisberger, a Balanchine pupil and friend. Balanchine ballets have constituted a key component of the troupe's repertory from the start; "Square Dance," created in 1957, is the most recent of these acquisitions, first performed by the Pennsylvanians last year. Peter Anastos' delectable spoof, "Yes, Virginia, Another Piano Ballet," points to the continuing company quest for interesting work by newer, younger choreographers (free-lancer Anastos was a founder of the male travesty troupe, Les Ballets Trockaderos de Monte Carlo, for which "Yes, Virginia" was originally done). Finally, the company's version of "Swan Lake," Act II, staged by Pennsylvania's artistic director Benjamin Harkarvy, is representative of the troupe's individual approach to traditional classics.

The relation of the choreography in "Square Dance," set to music by Vivaldi and Corelli, to the rusticity suggested by the title was always rather tenuous, but in its first form this was at least concretized by the presence of fiddlers and a caller on stage. Even these externals were pared away in a 1976 revision; what remains is a fascinating exercise in pure, abstract neoclassicism, with only the geometrical floor patterns and partnering maneuvers to evoke the ballet's inspirational origins. With Tamara Hadley and William DeGregory superbly heading a contingent of six couples, this Pennsylvania performance caught the work's speed, zest and formal intricacy to near-perfection. One section for the women features the steps called gargouillades--traveling sidewise jumps with an ornamental ripple, scintillating and extremely difficult. Hadley tossed them off with ease, her feet flashing and undulating like a school of minnows. DeGregory, quick and incisive in the virtuoso allegro passages, was also very eloquent in the archings and stretchings of the adagio Balanchine added in '76 for Bart Cook.

Anastos has made changes in "Yes, Virginia," too, including two sections added especially for the Pennsylvania production. The company gives this canny satire on neoromanticism several humorous twists of its own, as demonstrated by last night's adroitly scampering cast--Mimi Keith, Michelle Geryk, Linda Karash, Roger Triplett and Roy Kaiser. They understood that the witty secret of the ballet isn't slapstick so much as the deadly accuracy of Anastos' parody--the piece is so funny because the choreographer perceives the mannerisms of his model (Jerome Robbins' "Dances at a Gathering") so thoroughly and well.

Harkarvy's "Swan Lake" takes its liberties--as what production of the ballet does not?--but the result seems keenly attuned to the spirit of Lev Ivanov, whose innovations in the treatment of soloists and corps de ballet as an integrated dramatic unit gave the original Act II its special cachet. Last night's performance, with Melissa Podcasy as an impassioned, lyrical, introverted Odette and Edward Myers as her spellbound Siegfried, wasn't exactly overwhelming, but it remained deeply engrossing in its sustained consistency of mood and image.