Lexicalized Tree-Adjoining Grammars (LTAG) - ad-teaching.infor...

Substitution
Definition (Substitution)
Let γ be a tree containing a substitution node n labeled by X and
α an initial tree whose root node is also labeled by X.
By applying the substitution operation on (γ, n) and α, one gets a
copy γ ′ of γ in which n has been replaced by α. If γ, n, α do not
fulfill the above conditions, the operation is undefined.
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