import java.util.*;
class Tokens {
    int pos;
    String[] tokens;
    /*  */
    /* Convert the source into an array of strings. */
    Tokens (String source) {
	StringTokenizer st = new StringTokenizer(source);
	int tokenCount = st.countTokens();
	tokens = new String[tokenCount];
	for (int i = 0 ; i < tokenCount ; i++) {
	    tokens[i] = st.nextToken();
	}
    }
    boolean atEnd() {
	return pos == tokens.length;
    }
    boolean nextIs(String target) {
	return tokens[pos].equals(target);
    }
    String next() {
	return tokens[pos++];
    }
    void eat(String s) throws SyntaxError {
	if (pos != tokens.length && tokens[pos].equals(s))
	    pos++;
	else
	    throw new SyntaxError(s);
    }
}