Why do you need to define t2 simultaneously to 'a node and t1?

The following session is ok in Caml Light

>       Caml Light version 0.73

#type 'a node = {x: 'a; y: t1}
and t1 = A | B of t1*t1;;
Type node defined.
Type t1 defined.
#type t2 = C | D of (string * t2) node;;
Type t2 defined.
#type t2 = C | D of (string * t2) node | E of bool node;;
Type t2 defined.
#

or, if you prefer in Objective Caml

        Objective Caml version 1.05

# type 'a node = {x: 'a; y: t1}
a  nd t1 =   A | B of t1*t1;;
type 'a node = { x: 'a; y: t1 }
type t1 = | A | B of t1 * t1
# type t2 = C | D of (string * t2) node;;
type t2 = | C | D of (string * t2) node
# type t2 = C | D of (string * t2) node | E of bool node;;
type t2 = | C | D of (string * t2) node | E of bool node

V. Ménissier-Morain