TEST(ExistsMatchExpression, MatchesScalar) {
    ExistsMatchExpression exists;
    ASSERT(exists.init("a").isOK());
    ASSERT(exists.matchesBSON(BSON("a" << 1), NULL));
    ASSERT(exists.matchesBSON(BSON("a" << BSONNULL), NULL));
    ASSERT(!exists.matchesBSON(BSON("b" << 1), NULL));
}
TEST(ExistsMatchExpression, MatchesScalar) {
    ExistsMatchExpression exists;
    ASSERT(exists.init("a").isOK());
    ASSERT(exists.matchesBSON(BSON("a" << 1), NULL));
    ASSERT(exists.matchesBSON(BSON("a" << BSONNULL), NULL));
    ASSERT(!exists.matchesBSON(BSON("b" << 1), NULL));
}