= Solution
`Tree` is a <type constructor>, taking the element <type parameter> `a` to the <algebraic data type> `Tree a`. It is not a value and has no ordinary value-level <function type>; its <kind of a type> is `* -> *`. The other names are <data constructors> with <polymorphic> <type signatures>:
``
One :: a -> Tree a
Two :: Tree a -> Tree a -> Tree a
``
`One` constructs a <leaf-labelled binary tree> with one leaf; `Two` joins two such <trees> under a new internal node. Unlike a general <binary tree> representation with empty child slots, this representation has no empty tree.<Haskell 2010 Language Report>[§4.1.1, kinds; §4.2.1, algebraic datatype declarations]{ref}
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