doc: ad doc to SK function

Signed-off-by: Dr. Carsten Leue <carsten.leue@de.ibm.com>

function/bind.go

@@ -26,6 +26,7 @@ func Bind2nd[T1, T2, R any](f func(T1, T2) R, t2 T2) func(T1) R {
 	}
 }
 
+// SK function (SKI combinator calculus).
 func SK[T1, T2 any](_ T1, t2 T2) T2 {
 	return t2
 }

function/function.go

@@ -62,6 +62,7 @@ func First[T1, T2 any](t1 T1, _ T2) T1 {
 }
 
 // Second returns the second out of two input values
+// Identical to [SK]
 func Second[T1, T2 any](_ T1, t2 T2) T2 {
 	return t2
 }

tuple/tuple.go

@@ -21,10 +21,12 @@ func Of[T1 any](t T1) Tuple1[T1] {
 	return MakeTuple1(t)
 }
 
+// First returns the first element of a [Tuple2]
 func First[T1, T2 any](t Tuple2[T1, T2]) T1 {
 	return t.F1
 }
 
+// Second returns the second element of a [Tuple2]
 func Second[T1, T2 any](t Tuple2[T1, T2]) T2 {
 	return t.F2
 }