condition, sections

I am sure I did this before but I cannot find the code. Search didn't helped...
I have this

SectionGroup 1
Section 1a
Section 1b

Section 1b can only be selected when section 1a is selected.
So if 1a is unselected maybe to disable and unselect 1b ?
If 1a is selected, enable 1b ?

Little help please ?

Thanks,
OJi.

I found this thread.

SectionGetFlags ${sec3} $0

IntOp $0 $0 & ${SF_SELECTED}

StrCmp $0 ${SF_SELECTED} 0 skip

  SectionGetFlags ${sec2} $1

  IntOp $1 $1 | ${SF_SELECTED}

  SectionSetFlags ${sec2} $1

skip:

It's not working 100% as it should but it's OK for now.