[Caml-list] "this ground coercion is not principal"

[Martin DeMello <martindemello@gmail.com>]

What does this warning mean? I got it while compiling some code off
github [https://github.com/JoeDralliam/OcsfmlExamples/blob/master/Examples/sockets.ml]
and by experimenting it seems to be related to this bit from the
manual

Note that the coercion ( expr :>  typexpr ) is actually an abbreviated
form, and will only work in presence of private abbreviations if
neither the type of expr nor typexpr contain any type variables. If
they do, you must use the full form ( expr :  typexpr1 :>  typexpr2 )
where typexpr1 is the expected type of expr. Concretely, this would be
(x : N.t :> int) and (l : N.t list :> int list) for the above
examples.

but I could not find anything on google for either "ground coercion"
or "principal type coercion" that would explain what it was actually
saying. Could someone please point me to a reference?

martin