public class ArithBinary extends Binary {
  public Object sem (String op) throws SemanticException {
    super.sem();
    if (aT == Integer.class && bT == Integer.class) return T = Integer.class;
    if (aT == Float.class && bT == Float.class
        || aT == Integer.class && bT == Float.class
        || aT == Float.class && bT == Integer.class) return T = Float.class;
    throw new SemanticException("binary "+ op +" expects float or int");
  }
}