Def ij == j<i

is mentioned by

Thm* x,y:. xy  xy	[assert_of_le_int]
Thm* i,j:. ij = (j<i)	[bnot_of_le_int]

Try larger context: StandardLIB IF YOU CAN SEE THIS go to http://www.cs.cornell.edu/Info/People/sfa/Nuprl/Shared/Xindentation_hack_doc.html