Returns the crystal of letters of the given type.
For classical types, this is a combinatorial model for the crystal
with highest weight Lambda_1 (the first fundamental weight).
Any irreducible classical crystal appears as the irreducible component
of the tensor product of several copies of this crystal (plus
possibly one copy of the spin crystal, see CrystalOfSpins).
See M. Kashiwara, T. Nakashima, \textit{Crystal graphs for representations of
the $q$-analogue of classical Lie algebras}, J. Algebra \textbf{165} (1994), no. 2, 295--345.
Elements of this irreducible component have a fixed shape, and can
be fit inside a tableau shape. Otherwise said, any irreducible
classical crystal is isomorphic to a crystal of tableaux with
cells filled by elements of the crystal of letters (possibly
tensored with the crystal of spins).
INPUT:
T -- A CartanType
EXAMPLES:
sage: C = CrystalOfLetters(['A',5])
sage: C.list()
[1, 2, 3, 4, 5, 6]
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