I have 3 module types, each of which start out "the same", viz:
module type DETERMINANT = sig
  type 'a lstate
  type 'a tag_lstate = [`TDet of 'a lstate ]
  type ('b,'v) lm = ('a,'v,'s,'w) cmonad
    constraint 's = [> 'a tag_lstate]
    constraint 'b = 'a * 's * 'w
  ...
end
module type RANK = sig
  type 'a lstate = ('a, int ref) code
  type 'a tag_lstate = [`TRan of 'a lstate ]
  type ('b,'v) lm = ('a,'v,'s,'w) cmonad
    constraint 's = [> 'a tag_lstate]
    constraint 'b = 'a * 's * 'w
  ...
end
module type TRACKPIVOT = sig
  type 'a lstate
  type 'a tag_lstate = [`TPivot of 'a lstate ]
  type ('b,'v) lm = ('a,'v,'s,'w) cmonad
    constraint 's = [> 'a tag_lstate]
    constraint 'b = 'a * 's * 'w
 ...
end

[I realize that I will have to lose the RANK type's definition of 'a 
lstate when I abstract this out]
The only thing different between those 3 is the name of the polymorphic 
variant - so clearly an opportunity for abstraction!  But I can't seem 
to achieve this.  Is there a way?

Jacques