Computing Fibonacci numbers with the U combinator from Combinatory Logic, by kbk ,2012
namespace path ::tcl::mathop
proc U {x args} {tailcall apply $x $x {*}$args}
puts [U {{f n} {
if {$n <= 1} {
return $n
} else {
tailcall + [U $f [- $n 1]] [U $f [- $n 2]]
}
}} 10]