Who Cites let?
let	Def  let x = a in b(x) == (x.b(x))(a)
	Thm*  A,B:Type, a:A, b:(AB). let x = a in b(x)  B

Syntax:

let x = a in b(x)

has structure:

let(a; x.b(x))

About: IF YOU CAN SEE THIS go to /sfa/Nuprl/Shared/Xindentation_hack_doc.html