Hi,

Lists are polymorphic but they are uniform : elements of a list must have all 
the same type (which can be any type).

Hence, "mover" is a function that expects a list of objects, all of the same 
type, and which type contains a method "move".

Thus, with 
 q: p1d_1
 p: p1d_2

Expression "[q;q]" is of type "p1d_1 list" and "[p;p]" is of type "p1d_2 list",
thus "mover [q;q]" and "mover [p;p]" are well-typed.

But "[p;q]" is not well-typed.

Here, p1d_2 is subtype of p1d_1 : you can coerce p to have the type p1d_1.
Thus "[(p:>p1d_1);q]" is of type p1d_1 list.
And "mover [(p:>p1d_1);q]" is well-typed.

Sylvain.