Def Y(f) == (x.f(x(x)))(x.f(x(x)))

is mentioned by

Def fib(n) == if n=0  n=1 1 else fib(n-1)+fib(n-2) fi  (recursive)	[fib]
Def gcd(a;b) == if b=0 a else gcd(b;a rem b) fi  (recursive)	[gcd]

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