public static URepeated create(CharSequence identifier, UExpression expression) {
    return new AutoValue_URepeated(identifier.toString(), expression);
  }
@Test
  public void unifies() {
    JCExpression expr = parseExpression("\"abcdefg\".charAt(x + 1)");
    URepeated ident = URepeated.create("foo", UFreeIdent.create("foo"));
    assertThat(ident.unify(expr, unifier)).isNotNull();
    assertThat(unifier.getBindings()).containsExactly(new UFreeIdent.Key("foo"), expr);
  }