Def Y(f) == (x.f(x(x)))(x.f(x(x)))

is mentioned by

Def ncompose(f;n;x) == if n=0 then x else f(ncompose(f;n-1;x)) fi   (recursive)

[ncompose]

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