The notions of a matrix of trees and of a well formed matrix of trees are introduced. They arose from research in formal language theory. It is proved that each matrix of trees contains a “relatively large” submatrix which is well formed.
Ehrenfeucht, Andrzej and Rozenberg, Grzegorz, "A Note on Matrices of Trees ; CU-CS-044-74" (1974). Computer Science Technical Reports. 43.