private void adjustAbstractMethod(ClassSymbol c,
                                      MethodSymbol pm,
                                      MethodSymbol im) {
        MethodType pmt = (MethodType)pm.type;
        Type imt = types.memberType(c.type, im);
        pmt.thrown = chk.intersect(pmt.getThrownTypes(), imt.getThrownTypes());
    }