= Leaf-labelled binary tree
= Leaf-labelled tree
{synonym}
A <binary tree> with each data label stored at a <leaf of a tree>. One useful <Haskell> datatype has constructors `One a` for a leaf and `Two (Tree a) (Tree a)` for an internal node. Every finite inhabitant has at least one leaf, so this datatype cannot encode an empty leaf sequence. Its natural <tree fold> has a leaf operation `a -> b` and an internal operation `b -> b -> b`; replacing the two constructors by those operations defines the <structural recursion>.
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