is mentioned by

Thm* n,m:, f:(nm). Inj(n; m; f) nm	[pigeon-hole]
Thm* n,m:, f:((n-1)(m-1)). Inj((n-1); (m-1); f) Inj(n; m; f[n-1:=m-1])	[fappend-inject]

In prior sections: mb nat graph 1 1 fun 1

Try larger context: Graphs