Def  x is the u:A. P(u) == P(x) & (u:A. P(u)  u = x)

is mentioned by

Def  !x:A. P(x) == x:A. x is the x:A. P(x)

[exists_unique]

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