[isabelle] Combining locales

Sorry if I posted this question already a few month ago:

Having a locale with two parameters

 locale A =
   fixes X :: "'a"
   fixes Y :: "'a"

I want to derive a locale from that, where the type of the parameters is constrained, for example:
 locale B = A +
   constrains X :: "'a list"

this will result in the error message:
*** exception TYPE raised: unify_parms: failed to unify types ?'a and 'b
*** At command "locale".

 locale B = A +
   assumes "False ==> X = (X::('a list))"

as well as
 locale B = A +
   constrains X :: "'a list"
   constrains Y :: "'a list"

gives the expected result. Is there any reason for the first declaration to fail ? The second one is obviously a "dirty hack", while the 3rd one can be tedious to write down if having many fixed parameters with related types.

Greeting and thanks in advance for any hints
 Peter Lammich

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