Def Inj(A; B; f) == a1,a2:A. f(a1) = f(a2)  B  a1 = a2

is mentioned by

Def Bij(A; B; f) == Inj(A; B; f) & Surj(A; B; f)

[biject]

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