Def tree_con(E;T) == E+(TT)

is mentioned by

Thm* tree_con(E;Tree(E))  Tree(E)	[tree_subtype2]
Def Tree(E) == rec(T.tree_con(E;T))	[tree]

Try larger context: MarkB generic IF YOU CAN SEE THIS go to /sfa/Nuprl/Shared/Xindentation_hack_doc.html