bug#20351: isearch-lax-whitespace should be customizable variable

[Drew Adams]

> > > The technical term is "equivalence classes".
> >
> > I don't think so.  This term would apply equally well to case-
> > fold-search.
> 
> No, I don't think so.  Case equivalence and canonical/compatibility
> equivalence are separate aspects.

Are the latter not equivalence classes?  And the former?

In what way does ignoring letter case not put groups (typically
pairs) of chars into equivalence classes?